Modern Database TechniquesPart 1: Object Oriented Databases

4. Concepts of Object Oriented Databases

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Chapter 4: Overview

4.1 Type constructors, complex objects

4.2 Classes: Type- and Set-View

4.3 Relations between Classes, Aggregation

4.4 Object Identity

4.5 Inheritance

4.6 Query Operations

4.7 OQL Object Query Language of ODMG

4.8 Methods

4.9 Integrity

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Requirements for OODBS

object-oriented perspective Complex objects Object identity Encapsulation Types and classes Class hierarchies Overloading of methods

optional: Multiple inheritance

Database perspective Persistance Disk-storage organisation Concurrent transactions Recovery mechanisms Query languages Database operations

optional: Distributed databases Version administration

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4.1 Type constructors, complex objects

To be able to store many objects, a set constructor is necessary

Type of a relational tableSET OF (TUPLE OF (Typ1 A1, . . . , Typn An))

In OODBS user defined classes can be used besides basic types.

TUPLE OF corresponds aggregation. There are different implementations of the set

operator in different OODBMs

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Overview Type Constructors

Tuple constructor TUPLE OF Combines several components of possibly different type Corresponds to aggregation

Set constructor SET OF Many elements of the same type build a set. Each element can only be contained once in the set.

Multi-set constructor BAG OF: like set but one element can have copies in the bag

List constructor LIST OF: like bag, but Sequence is of interest

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Overview Type Constructors (cont.)

Array constructor ARRAY OF: like list, but may contain gaps index of type long integer

Dictionary constructor DICTIONARY OF (key, value) unordered sequence of

(key, value)-pairs lookup operation

returns value to a given key

Type constructors are used recursivly

0 1 2 3 …

Atlanta Paris London

Key Value

A excellent

B very good

C good

D satisfactory

E sufficient

F fail

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Example: Type Construction for PersonsGraphical Representation

List

Tuple

Set

string

SetTuple

Tuple

date

stringstringstringstring

FirstName

SecondName

LastName

Title Salutation

DOB

Tuple

string stringstringstringstring

Street City State ZIP Typestringstring

PhoneNr

Type

Name

Persons

Person

Addresses

Address

PhoneNumbers

PhoneNumber

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Example Type Construction in O2: Persons

SET OF TUPLE OF (Name: TUPLE OF ( FirstName: STRING, SecondName: STRING LastName: STRING, Title: String …),Adresses: SET OF TUPLE OF ( Street: STRING, City: STRING, ZIP: STRING, State: STRING, Type: String),PhoneNumbers: LIST OF TUPLE OF ( PhoneNr: STRING, Type: STRING),DOB: DATE)

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ODMG Standard

ODL (object definition language) for ODMG-types extension of IDL (interface definition language) similar to C++ but language independent

All classes inherit from interface Object that provides constructor function (interface ObjectFactory) locking information and operation identity comparison function same_as(object) copy function delete function

Different constructors for collection types

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ODMG Standard (cont.)

Collections supported by the ODMG-Model

Set <t> Bag <t> List <t> Array <t> Dictionary <t, v> where t and v are any types

Collections supported for objects and for literals

Atomic types long, long long, short … float, double boolean, octet char, string enum

type generator can only take values listed in

declaration

Structured literals date time, timestamp interval user defined with struct

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Example: Persons in ODMG-Model

Class Person (extent Persons){attribute NameCl Name; attribute date DOB; attribute set <Address> Addresses; attribute list <PhoneNr> PhoneNrs;}Class NameCl{attribute string FirstName; attribute string SecondName; attribute string LastName; attribute string Title; attribute string Salutation;}

no own set of objects

set of objects

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Example: Persons in ODMG-Model

Class Address{attribute string Country; attribute string City; attribute string Street; attribute string ZIP; attribute string State; attribute string Type;}Class PhoneNr{attribute string PhoneNr; attribute string Type;}

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Type Construction with UML

Tuple ConstructorSet Constructor

List or Array Constructorcomplex properties

No dictionary constructor

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Type Constructors in Caché

Tuple: Properties of a class Set:

Class automatically gets extent if it inherits from %Persistent

No set-operator for components:Property x As set Of Y; not possible

Use list- or array-constructor instead, or relations Bag: not supported List: list Of

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Type Constructors in Caché

Array: not directly supported, use dictionary with integer key values

Dictionary: array Of Array in Caché is Dictionary in ODMG-Model !

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Example: Persons in Caché

Class Person Extends (%Persistent){Property Name As NameCl; Property DOB As %Date; Property Addresses As list Of Address; Property PhoneNumbers As list Of PhoneNumber;}Class NameCl Extends (%SerialObject){Property FirstName As %String; Property SecondName As %String; Property LastName As %String; Property Title As %String; Property Salutation As %String;} back

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Example: Persons in Caché

Class Address Extends (%SerialObject){Property City As %String; Property Street As %String; Property ZIP As %String; Property Country As %String; Property State As %String; Property Type As %String;}Class PhoneNumber Extends (%SerialObject){Property PhoneNr As %String; Property Type As %String;}

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Type Construction: Saving Accounts

Practice: Define the object type of Saving Accounts as complex type graphically: Different types: fixed term, open term, compound interest Interest rate may increase every year: 2% - 3% - 3.5% Transactions on account

Define the same object type in Caché What are the consequences of recursive types,

e.g. if Person contains a set Friends of type Person?

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Type Construction for Saving AccountsGraphical Representation

List

Tuple

Set

Arr.datestring

InterestRate

AccountNumber

Year

Begin

Tuple

floatdate

Date Amount

Index:

Saving Accounts

Saving Account

Interest

Transactions

Transaction

stringType

date

End

int

float

string

Sender

string

Purpose

Value:

float

CurrentAmount

redundant information

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Operations on Complex Types

Tuple constructor Access to components Test two tuples on equality

All collection operators Test if collection is empty Test if collection contains a certain element Access to all elements (iterator) Insert, update, or delete element

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Operations on Complex Types (cont.)

Set constructor Set comparisons: =, Set operations: intersection, union, difference

Bag constructor Number of occurencies of an element Set operations: intersection, union, difference No set comparisons

List construktor Access in sequence of list Insert at certain position or after certain element Concatenation of lists

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Operations on Complex Types (cont.)

Array constructor Access to elements at certain index

Dictionary constructor Access to elements at certain key Test whether certain key is in dictionary Bind a value to a certain key (insert) Unbind a value from a key (delete)

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Operations on Complex Types in Caché

Persistent classes (tuple and set constructor) Access to components by dot-syntaxp1.Name.FirstName if p1 is a Person object

Test if object exists: class methods %Exists(oid) and %ExistsId(id)

Retrieve stored objects into memory: class methods %Open(oid) and %OpenId(id)

Insert or update an element: %Save()-methodp1.%Save()

Delete an element: class methods %Delete(oid) and %DeleteID(id)

Iterator to access all objects of a class: query Extent()

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Methods for List- and Array- (Dictionary-) Collections

Method Purpose Success Failure

List: Insert(value)

InsertAt(value, index)

Array: SetAt(value,key)

Add value to collection

1 error status

List: InsertList(list) Appends list 1 error status

Find(element) Tests whether element is in list

index or key

""

GetAt(key) Get value at key value ""

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Methods for List- and Array- (Dictionary-) Collections (cont.)

Method Purpose Success Failure

RemoveAt(key) Remove object key

value at key

""

Count() Count items integer

Clear() Empty collection 1 error status

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4.2 Classes: Type- and Set-View

Class has different meanings: Set of objects belonging together Container for all hitherto created objects of the class Type = construction schema of the objects Domain for (abstract) objects

A state function maps each object to its value, an element of the domain.

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Concepts for Classes: Type-based

class Line

Point begin;Point end;int width;

Line a2

(3;0);(5;17);

2;

Line yxa

(3;15);(1;1);

6;

type of

A class defines a complex type Objects of the class are not gathered in a container This is the situation in programming languages like C++

Line x

(3;15);(5;17);

6;

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Concepts for Classes: Set-based

Line x

(3;15);(5;17);

6; Line a2

Line gt

"p1";"p2";

"thin";

A class is a container Types of the elements are not fixed

4 1

5 2

6 3

x k

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Set-type-based

class Line

Point begin;Point end;int width;

Line x

(3;15);(5;17);

6;

Line a2

(3;0);(5;17);

2;

Line yxa

(3;15);(1;1);

6;

class defines a complex

type

AND class is container for objects (extent)

This is the solution in OODBMSs

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4.3 Relations between Classes

Every class consists of attributes(= properties = components)

Their types can be atomic types or classes. Class-componentclass-relations implement

relationships between different classes Distinguish: class-subclass-relation → see 4.5 Different semantics of component classses

shared/private component objects dependent/independent component objects encapsulated/standalone component objects unidirectional/mutual component relation

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Shared/Private Component Objects

Shared component objects: A component object is component of many objects An object can be component of objects of different

classes M:N(1)-relation between component class and class Example: Branch is shared component of ATM

Private component objects: Each component object is component of only one object 1:N(1)-relation between class and component class Example: ATM is private component of Branch

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Dependent/Independent Component Objects

Dependent component objects only exist with object they are component of are created/deleted with object they are component of Shared dependent objects are deleted with last object

they are component of Example: Name is dependent of Person

Independent component objects can exist without another object are created and deleted independently If you delete a component object, take care of objects

which have it as a component. Example: Branch is independent component of an ATM

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Encapsulated/Standalone Component Objects

Encapsulated component objects are only accessible by their superior objects. are always dependent Redundancy when used for M:N-relations Example: Name is encapsulated in Person

Standalone component objects are accessible independently or

by the objects which have them as component. Example: Branch is an standalone class. A branch is

accessible without an ATM- or employee-object

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Unidirectional/Mutual Component Relation

Unidirectional component relation One object is obviously the superior object, the other the

component No direct way from the component to its superior object Example: Person → Name, Car → Motor

Mutual component relation Each of two objects has the other one as component. Implements bidirectional relationship between two

classes Example: Branch ↔ ATM

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Different Types of Component-Classes

Practice: Decide which is the type for the following examples of classes and component classes:

shar priv enc aut dep ind dir

Buildings – Rooms

Countries – Towns

ATMs – Currency Orders

Lectures – Students

Books – Publishers

Books – Pages

Folders – Files

Parents – Children

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Different Possibilities for Relationships:a) Encapsulated component class

Complex objects with encapsulated objects of component class

Suitable for für 1:1- and 1:N-relations Redundancy when used for M:N-Relations Access not symmetrical:

Easy from object to component Difficult from component to parent object See example Person NameCl is encapsulated in Person because it inherits from

%SerialObject

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Different Possibilities for Relationships: b) Mutual Component Classes

Two classes have each other as component Can be used for 1:1-, 1:N- and M:N-Relations Symmetrical access System must ensure symmetry The relation must not have attributes of its own. E. g. the

examination results of students in the relation "Students attend Lectures" cannot be represented

Possibility c) → later

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Relationships in Caché

A relationship is a two-way link between an object of one class and several objects of another (possibly the same) class.

Relationship types: Independent (One-to-many). Dependent (Parent-to-children).

Children stored together with parent However not encapsulated (children get own object identifiers) Classes must be different in this case.

Indirectly supported: One-to-one. Not supported: Many-to-many, use connection class

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Relationships in Caché (cont.)

Relationship specified in both class definitions. Relationship statement:

specialized Property statement. One, many, parent, and children define cardinality of

each side of relationship.

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Example: Branch and ATM Class Model

FCE.ATMBranchCurrencyBalances1

0..*PhoneOpensClosesATMs

FCE.Branch

StreetCityStateZip

FCE.RealEstate0..1

1

1

0..1

Practice: Discuss the possibilities to implement the relation "Branch has ATMs"

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Cardinality of Relationships in Caché

Cardinality: number of elements on each side of a reference.

Describe cardinality in terms of classes. “FCE.Branch has a parent-to-children relationship with

FCE.ATM. Multiple ATMs belong to each branch.” Specify cardinality in terms of properties.

“The ATMs property of the FCE.Branch class has cardinality/type Children. The Branch property of the FCE.ATM class has cardinality/type Parent.”

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OOP Relationship Advantages

Relationship is a combination of a reference and a list collection. Performance degrades as number of objects on

many/children side surpasses 900-1000. In v4.1, use of CACHETEMP reduces degradation.

When classes are persistent, relationship provides referential integrity for object deletions on one/parent side.

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SQL Relationship Advantages

When classes are persistent: Many/children table has foreign key from one/parent

table. Relationship provides referential integrity for row

deletions on one/parent side.

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Referential Integrity

For relationships between persistent classes from A to B

One-to-Many: Deletion of an object of class A fails as long as it points

to at least one object of class B. To delete an object of class A, delete the references to it

from objects of class B. Parent-to-Children:

Deletion of an object of class A deletes all related objects of class B.

Same is true for rows in tables.

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Demonstration: Create a Relationship

Create two classes. Add a property to one class.

Specify name. Use plural name for property with cardinality many/children, and

singular name for property with cardinality one/parent. Specify property as a relationship. Specify cardinality: One, Parent, Many, Children. Specify type: reference second class. Specify name of relationship property to be added to

second class.

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Caché: Simulate One-to-One Relationship

Create one-to-many relationship. Change index added on property with cardinality =

one (in “many” class) to unique. Results:

Prevents object on “one” side from being referenced by more than one “many” object.

Prevents deletion of object on “one” side. But doesn’t prevent deletion of object on “many” side.

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Using Relationships

Link objects in two equivalent ways: Insert many/children into one/parent collection. Reference one/parent from many/children object.

No control over ordering of many/children in one/parent collection. To control ordering, use list- or array-constructor

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Using Relationships (cont.)

From one/parent side, many/children side is a collection. To link, either: Insert many/children object into one/parent object’s

collection:do branch.ATMs.Insert(ATMOne)

Insert many object ID directly into one object’s collection, without opening object (only for one-to-many):do branch.ATMs.InsertObjectId(atmid)

Insert new many/children object into one/parent object’s collection:do branch.ATMs.Insert(##class(FCE.ATM).%New())

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Using Relationships (cont.)

From many/children side, one/parent side is a reference. To link, either: Reference one/parent object from many/children object:

set ATMOne.Branch = branch Set one/parent object ID into many/children object

directly, without opening one/parent object, using PropertyNameSetObjectId() method.do ATMOne.BranchSetObjectId(branchid)

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Storing Relationships

Calling %Save() on either one/parent or many/children triggers save of entire relationship.

IDs of Children in Parent-to-Children relationship depend on parent ID. Format: parentID||childID.

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Implicit Joins

In a query, using -> operator on a foreign key column allows direct references to columns in foreign table.

Syntax: table.foreignkey->column

Joins: -> operator in SELECT clause creates LEFT OUTER

JOIN. -> operator in WHERE clause creates INNER JOIN.

Transmits efficiency of OO-model to SQL

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Implicit Joins (cont.)

May be chained.select Branch->Address->Street

Options when querying tables of a parent-to-children relationship: childtable.parentforeignkey->columninparenttable parenttable.childtable->columninchildtable

Works even though childtable is not a column in parenttable. Specify childtable without schema.

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Practice: Query with -> Operator

Write a query with join-conditions, that finds the name and street of all branches where the phone-number begins with 6 together with the streets of all ATMs

Define 2 queries for this task using the -> Operator starting at the Branch-table starting at the ATM-table

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Different Possibilities for Relationships: c) Connection Class

Employee Customer

subject area

0..*0..*advises

advisers

relation with attribute

The relation becomes an own class Suitable for M:N-relations with attributes of their own Two 1:N-relations from each class to connection class In Caché necessary for every many-to-many relation

Example: Employees advise customers Different employees advise customers in different areas

like loans, investments, or online banking

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Different Possibilities for Relationships: c) Connection Class

The relation becomes an own class Suitable for M:N-relations with attributes of their own Two 1:N-relations from each class to connection class In Caché necessary for every many-to-many relation

Example: Employees advise customers Many employees advise many customers in different

areas like loans, investments, or online banking

Employee Customeradvises

advisers0..* 0..*Customer

contacts1 employee

customer

subject area

attribute

connection class

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Summary: Different Possibilities to Implement Relationships in Caché

Component class %Serializable Use for unidirectional, private, dependent, encapsulated

components Component class %Persistent

Use for unidirectional, private, dependent, standalone components Relationship one-to-many

Use for bidirectional, private, independent, standalone components Relationship parent-to-children

Use for bidirectional, private, dependent, standalone components Connection class

Use for bidirectional, shared, independent, standalone components Relations with own attributes

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4.4 Object Identity Disadvantage of Keys

No difference between update of an objects value (content) and its identity (key), e. g. name of a branch

Problem, when many objects contain the same component object (e. g. ATMs of a branch)

It is not possible to have different objects with the same key value.

Queries require inefficient joins.

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Types of Object Identity

Sum of all values, not practical because: Values can change Object in DB only contains subset of values of real object

Values of characteristic attributes E. g. fingerprint or DNA for real persons Difficult to store in DB Not applicable for all classes,

e. g. chairs in a lecture room

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Types of Object Identity

Physical addresses, direct references, pointers Pointers point to component-objects Pointers point to begin of a list for set attributes

User defined names of a name domain e. g. license numbers of cars

Automatic keys = surrogate keys = identifier attributes (implementation obvious)

Abstract objects (implementation hidden to user)

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Distinguish Objects and Values

Objects Example:

Branch "City Hall" are not printable must be created and

defined carry no information

itself are described by the

values of their attributes

Values Example :

number 182 printable already exist

carry information themselves

describe objects

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Object Identity by Physical Addresses

Example: Identify buildings by their Addresses Advantages

simple implementation efficient access of components address is not a value of the object

Disadvantages physical data independence violated object cannot be moved Delete an object. Is physical address reusable?

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Object Identity by Names

Example: Names of persons Advantage

Name can be structured, easy to read: first- last name. Disadvantages

can be interpreted as value Name can change (e. g. marriage) Uniqueness must be checked → keys!

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Object Identity by Surrogate Keys

Examples: AutoNumber in Access, SEQUENCE in Oracle

Advantages Uniqueness guaranteed by system cannot be changed carries (nearly) no information, is substitute for object

Disadvantages Foreign key constraints must be defined Surrugate keys can be interpreted as values.

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Example: Saving Accounts with Identifier-Attributes

SET OF (TUPLE OF (Account_Nr: IDENTIFIER,

Type: STRING,

Begin: Date,

End: Date

Account_Holder: IDENTIFIER,

…

Transactions: LIST OF(TUPLE OF(Trans_ID: IDENTIFIER,

Amount: INT

…))))

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Saving-Accounts with Identifier-Attributes

Account_Nr

Type Begin End Account_Holder

… Transactions

172963 variable

12. JUN 2002

54355 1; 456 $; 1.7.02;…

2; -63 $; 5.7.02;…

3; …

134584 incr. interest

1. AUG 2005

31. JUL 2009

37704 1; 456 $; 1.8.05;…

2; 11 $; 31.12.05;

3; …

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Object Identity with Abstract Objects

Objects are elements of a not structured, countably infinite, abstract set with the only information that

the elements are different Operations on objects

Create new object Delete object Test on identity You never can print or see

an object itself or its Id

Abstract objects can be implemented (more or less well) by

physical addresses Names Identifier-attributes

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Hugo Ross1 May 89

Graphical Representation using State-Boxes

5 Garden AveSan Francisco, CA

88250

State of object 19

19

2

object 19

State of object 2

Representation is difficult, since objects cannot be printed.

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Abstract Objects

Advantages independent of the implementation theoretically sound foreign key constraints automatically guaranteed

Disadvantage implemented only in very few real OODBMS

Two versions One infinite set with abstract objects for all classes Disjoint domains of abstract objects for each class

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Saving-Accounts with Abstract Objects and Disjoint Domains

1 is not the name of an account holder, not the key of an account holder, but the whole object of type customer.

Account

Acc.Nr Type Beg. End Acc.Holder

… Transactions

1172963 varia

ble12. JUN 2002

1 τ1; τ2 ; τ3 ; …

2 134584 incr. interest

1. AUG 2005

31. JUL 2009

1τ4 ; τ5 ; τ6 ; …

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Operations on Objects

Create new or delete existing object Dereference objects to get their values Test on identity: o1 == o2

e. g. father of Peter == father of Susan Test on shallow equality (of values) Test on deep equality (of values) Assignment: create a reference to an object and

assign it to a variable Create shallow copy Create deep copy

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Comparison of Objects

Philosophical question: When are two objects identical?

Different comparisons Test on identity: Do two object-references refer to the

same object? Test on equality: Have two possibly different objects the

same values? If objects have component objects, compare whether the

component objects are identical: shallow equality or whether the component objects have the same

values: deep equality Similar questions when copying an object

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Practice: Operations on Objects

Create a new saving-account 3 by shallow copying 1

Create a new saving-account 4 by deep copying 1

Test on shallow equality of 1 and 3?

Test on deep equality of 1 und 4?

Assign 1 to variable MyPrimaryAccount

1 == MyPrimaryAccount?

1 == 3? Why are private component-objects a problem for

shallow copying?

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Object Identity in Caché

Difference between oref An oref is a temporary

identifier for an object in memory.

Remove object from memory → oref is no longer valid

cannot be printed

and object ID An object ID is a permanent

identifier for a stored object is automatically created by

%Save()-method never changes can be used to locate or

delete an object in the DB can be printed

Every class gets %New()-method to create objects in memory and %Save()-method to store them permanently in DB

Caché guarantees correspondence of orefs and object IDs

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Object Identity in Caché (cont.)

An object Id behaves like an abstract object, because it … is added as property for every class is automatically created when an object is saved for the

first time cannot be changed by the user

An object Id behaves different from an abstract object, because it … only exists for stored objects can be printed can be used to retrieve a stored object into memory carries information whether the object is encapsulated

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Object Identity in Caché (cont.)

IDs are class specific; ObjectId = Id + ClassName Caché does not provide functions for

testing shallow equality testing deep equality

If needed these functions must be programmed Create a copy of an object with the method

%ConstructClone() deep copy with parameter deep = 1 shallow copy with parameter deep = 0 private component-objects are cloned even with

parameter deep = 0; to avoid this, use deep = -1

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4.5 Inheritance

Special relation between objects of different classes "is a", e. g. Customer Miller is a Person

Type inheritance: Let T_super and T_sub be types.T_super is super type of T_sub, T_sub subtype of T_super for atomic types if T_sub T_super

e. g. short int long int for tupel-types, if T_sub has at least all components of

T_super hat, e. g. Customer and Person for set-types, if the element-type of T_sub is subtype of

the element-type of T_super

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Inheritance (cont.)

Class (set) inheritanceLet C_super und C_sub be classes C_sub is more special than C_super, if

set of objects (C_sub) set of objects (C_super) C_sub is subclass, C_super is superclass

Class and type hierarchy fit together, e. g. Class hierarchy: Customers Persons Type hierarchy: Customer has all attributes of Person

and additional ones Customer is subtype of Person DBMS guarantees that every object in class Customer is

in class Person, too. Define type inheritance, get set inheritance for free

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Operations for Inheritance

Specialization defines subclasses by inheritance of a superclass Not all objects of a superclass must be elements of one

of its subclasses. Generalization

combines classes to a collective superclass All objects of all subclasses become elements of the

superclass Often used to define abstract superclasses to simplify

programming, e. g. generalize Branch and ATM to CashLocation

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Class Tree and Class Graph

Specialize subclasses class tree

animals

vertebrates

mammals birds

invertebrates

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Addresses

Class Graph

A subclass inherits from more than one superclass (multiple inheritance) class graph

Not allowed in all object oriented systems

Supported by Caché

Person

Name: StringDOB: DatePhone: StringEmail: String

Employee

Salary: FloatESince: DateQualification: String

Customer

Type: StringCSince: DateRating: String

EmpCust

InterestBonus: Float

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Inheritance in ER-Diagrams

Example: ER-Diagram of a personal address-book

Person

FriendColleague

o

Name PhoneAddress

RoomNr WorkPhone HobbiesDOB

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Problems

An object cannot be element of different classes Store a colleague who is also your friend

new class ColleagueFriend necessary Explosion of combination classes

Class with n subclasses 2n - n - 1 combination classes Objects can't change their class to a subclass

E. g. colleague becomes friend Object must be deleted and created anew

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Inheritance in Caché

Single or multiple inheritance Overriding of properties or methods possible

To override a property, method argument, or method return value datatype, new datatype must be original datatype or its subclass.

To override a method, new method may have more arguments than original method, but not fewer.

Abstract Classes (objects only in subclasses) Syntax: Class Subclass Extends SuperclassClass Subclass Extends (SC1, SC2, …)

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Multiple Inheritance in Caché

Caché supports multiple inheritance.Class FCE.ATM Extends (%Persistent, %Populate, %XML.Adaptor)

In superclass list, order is important. First superclass provides class parameters and

members to subclass. Other superclasses provide class members only. Member name conflicts resolved in favor of superclass

listed latest (reverse order of superclass list).

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Multiple Inheritance in Caché (cont.)

Additional superclasses can provide: Complete methods for usage by subclasses. Method generators to generate code specific to

subclass. Empty methods for subclasses to implement.

Similar to Java's Interface concept.

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Abstract Classes

Even though a superclass can define properties and methods, it might not be meant for object instantiation. To prevent object instantiation from superclass, set

Abstract attribute to True. New Class Wizard also allows creation of an abstract class.

Application code instantiates objects from concrete (non-abstract) subclasses.

Suitable for Generalization, if the superclass only is created to

simplify programming disjoint partition of a superclass in subclasses

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Discussion of Abstract ClassesExample: Person Class

Is FCE.Person abstract? If yes, application code can’t create a Person object;

only Customer and Employee objects. If no, application code can create Persons, Customers,

and Employees. If abstract, FCE.Person class contains properties

and methods common to all types of persons. FCE.Customer and FCE.Employee concrete subclasses.

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Abstract Methods

Methods of an abstract superclass may optionally contain code to be inherited.

Setting a superclass method’s Abstract attribute to True shows that method doesn’t contain code. Subclasses should override method and provide code

particular to the subclass. Application code can’t call an abstract method.

Similar to Private.

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Practice: Create Class Graph

A car dealer not only sells cars but also journeys. Create a class graph with UML for the following

classes: Vehicle Car Truck Journey Person Customer Employee Sale

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4.6 Query Operations

Class specific operations: Each class has its own methods. A programmer has to implement these methods. Disadvantages:

A lot of work to do. It is not possible to decide whether set of operations is complete. Possibly bad performance.

Generic Operations: General database operations provided by the system generate class specific operations for every class Disadvantage: Encapsulation of objects violated

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Generic Query Operations

Operation Object oriented concept Projection Supertype Selection Subclass (Subset) Join Subtype of two types Union Superclass (Superset) Intersection Subclass of two classes Difference Subclass Grouping Degrouping Object navigation

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Grouping

Properties of a relation are combined to a new property. The new property is a set. This can be interpreted as nested relation.

Degrouping is the opposite operation.

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Example for Grouping: Situation before Grouping

ATM Branch Brand SerialNr Address CurrencyBalances

15 3 UBX 123-5 102Euro 3075USD 4788…

16 4 UBX 123-7 370Pound 2350USD 5340…

17 3 Abam 112675 20Rupees 3900USD 3340…

18 4 T.C.C. 45-44-6 876Euro 5150USD 2500…

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Example for Grouping: Situation after Grouping by Brand

Brand ATM Branch SerialNr Address CurrencyBalances

UBX 15 3 123-5 102Euro 3075USD 4788…

16 4 123-7 370Pound 2350USD 5340…

Abam 17 3 112675 20Rupees 3900USD 3340…

T.C.C. 18 4 45-44-6 876Euro 5150USD 2500…

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Object Navigation

If you have a certain object, you can directly access related objects.

Use dot-syntax. Example: Print the street names of all ATMs

belonging to the branch in object brchMainfor i=1:1:brchMain.ATMs.Count() { write ! brchMain.ATMs.GetAt(i)

.Address.Street}

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Result Sets in Class Hierarchy

Results of a query are organized in result sets. An iterator provided by the result set allows to

step through the elements of the result set. Three possibilities for the elements of result sets:

a) Relational operation: The elements of the result set are no objects, the result set is a pure relation.

b) Object generating operations: Generate new objects of a new abstract class.

c) Object conserving operations: The elements of the result sets are the existing objects.

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Example: Projection of ATMs to Brand and SerialNr

Brand SerialNr

UBX 123-5

UBX 123-7

Abam 112675

T.C.C. 45-44-6

New-Obj Brand SerialNr

15 UBX 123-5

16 UBX 123-7

17 Abam 112675

18 T.C.C. 45-44-6

a)

c) ATM Brand SerialNr

15 UBX 123-5

16 UBX 123-7

17 Abam 112675

18 T.C.C. 45-44-6

b)

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Degrouping cannot be object conserving

ATM Branch Brand SerialNr Address CurrencyBalances

15 3 UBX 123-5 102Euro 3075

15? 3 UBX 123-5 102USD 4788

16 4 UBX 123-7 370Pound 2350

16? 4 UBX 123-7 370USD 5340

17 3 Abam 112675 20Rupees 3900

17? 3 Abam 112675 20USD 3340

18 4 T.C.C. 45-44-6 876Euro 5150

18? 4 T.C.C. 45-44-6 876USD 2500

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Queries in Caché

Caché uses SQL for queries with extensions for object oriented access: Implicit join operator "->" Calculated properties Encapsulated components are automatically unfolded as

columns in the corresponding table in the form ParentTableColumn_ComponentColumn e. g. Name_FirstName in Person

Advantages: Well known standard. Object view by implicit join operator.

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Defining Class Queries

Class queries are SELECT queries only, saved with class. New Query toolbar button provides wizard to help build

proper SQL statement. Build complex statement by hand.

Query can use input parameters, to be supplied at runtime. For example:where amount > :variable Host variables replaced with value of parameter when

query runs.

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Class Queries (cont.)

You can use relational or object conserving operations, depending whether the object id is retrieved by the query.

Code generated when class is compiled. %ResultSet class provides OOP access to any

class query. ODBC/JDBC access as Stored Procedure. Every persistent class has Extent class query that

returns every ID in extent. Add EXTENTQUERYSPEC class parameter to specify

additional columns for Extent query.

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Example: Class Query

Retrieve all branches with ATMs in the area with a certain ZIP code

Query HasATMinZIP(ZIP As %String) As %SQLQuery(CONTAINID = 1)

{ SELECT %ID,Name,Phone,ATM->Address->Zip FROM Branch WHERE ATM->Address->Zip = :ZIP}

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Practice: Create Class Query

Create a class query to find all ATMs that have a certain currency, e. g. Indian Rupees, in their CurrencyBalances currency is a parameter Use Implicit join operator →

(for more information refer to slide Implicit Joins) You have to use the SQL-Name of the property

CurrencyBalances: ATM_CurrencyBalances The currencies' names are stored in the column

element_key of ATM_CurrencyBalances

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Using the Predefined Query Extent() Example: Branch

Create new result setSet rset = ##class(%ResultSet).%New

("FCE.Branch:Extent") Execute query: Do rset.Execute() Step through result setWhile (rset.Next()) { set br = ##class(FCE.Branch).%OpenId

(rset.Data("ID")) if br.Closes < PIECE($HOROLOG,",",2) {write br.Name _ " already closed",!}}

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Class Queries used as Views

Object conserving operations define virtual classes Example: Select all ATMs manufactured by UBX;

call the class query UBX-ATM. UBX-ATM is subset of ATM. UBX-ATM is automatically updated when ATM is

updated. Virtual classes correspond to VIEWS in the

relational model. Object conserving operations avoid explosion of

new classes.

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4.7 OQL Object Query Language of ODMG

Superset of SQL SELECT Differences and enhancements to SQL:

OQL is orthogonal i. e. the result of a query, operator or function can be used as part of other queries as long as the types fit.

A dot (".") or an arrow (“") can be used to access components (implicit join)

Access to methods OQL not only can query sets but also other structures:

bags, lists, arrays, and dictionaries Constructor or destructor operations instead of insert or

update respectively.

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Overview of OQL

Syntax structure of an OQL select-querySELECT defines data to be retrieved and structure of data

FROMspecifies collections referenced by the query and variables to iterate over those collections

WHERE specifies conditions for selecting objects Following slides present examples how to use OQL More information

R.G.G. Cattell. The Object Database Standard: ODMG 2.0. Morgan Kaufmann, San Francisco, California, 1997.

http://www.openquasar.de/de/components/quasar_persistence/oql.html http://www.eas.asu.edu/~cse494db/notes/oodbNotes/odmg_oql_mar2002.ppt

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OQL and PATH EXPRESSIONS Navigation Through Objects

Path expressions are used to navigate from one object to another.

Use dot (".") or arrow (“") Example:SELECT p.Name.LastName

FROM Persons p

WHERE pNameTitle = "Dr." Equivalent: FROM p IN Persons Persons is extent of persistent class Person Result is a multiset of literals; type Bag <string>

DISTINCT

set Set

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OQL:Traversing Attributes and Relationships

OQL does not allow path expressions to traverse over multivalued attributes and relationships.

Example: not allowedSELECT b.Name,b.ATMs.Address.StreetFROM b IN BranchWHERE b.ATMs.Address.ZIP = '022456'

Use instead variable a for ATMs

a

a, a IN b.ATMs

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OQL Extracting from Lists and Arrays

OQL provides an explicit notation for extracting the ith element from lists and arrays.

The first element in any indexed collection is assumed to be 0.

OQL also provides the operations: first and last Examples:

LIST(a,b,c,d)[1] bLIST (a,b,c,d)[1:3] LIST (b,c,d)FIRST(LIST(a,b,c,d)) aLAST(LIST(a,b,c,d)) d

Convert a list to a setLISTTOSET(LIST(1,2,3,2)) {1,2,3}

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OQL: Access Single Elements and Complete Extent

The ELEMENT-operator converts a set with one element into this element : ELEMENT( SELECT a FROM a IN ATM WHERE a.SerialNr = '543-346-XX')

Select all objects of class Person: Persons

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OQL: Embedded Queries

Example: SELECT a.Address.StreetFROM a IN (SELECT a

FROM Branch b, a IN b.ATMsWHERE b.Opens < '08:00:00'

WHERE a.Brand = 'T.C.C' Subquery also possible in SELECT- and WHERE-

part

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OQL: Structuring Results and Grouping

GROUP BY provides explicit reference to the collection of objects within each group or partition.

Example: SELECT STRUCT(Manufacturer, ATMSerialNr: (SELECT p.a.SerialNr

FROM p IN PARTITION))FROM a IN ATMGROUP BY Manufacturer: a.Brand

Result type: Struct <string, Set <string>>

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OQL: Aggregate Functions

OQL provides aggregate operators (min, max, count, sum, avg) over a collection.

Example: SELECT STRUCT(BranchObj: b,

NrATMs: COUNT(b.ATMs))FROM b IN Branch

Result type: Struct <Branch, integer>

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OQL Quantification

Existential quantification: exists x in e1: e2 TRUE if at least one element of e1 satisfies e2 Example: Select Branches with at least one ATM from

UBX?

SELECT b FROM Branch b WHERE EXISTS a IN b.ATMS: a.Brand = 'UBX'

Universal quantification: for all x in e1: e2 TRUE if all elements in e1 satisfy e2 Example: Select Branches with ATMs only from UBX SELECT b FROM Branch b WHERE FOR ALL a IN b.ATMS: a.Brand = 'UBX'

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Practice:Define OQL Queries

Find all ATMs which can supply at least 1000 "Indian Rupees".

Find all managers with the number of employees they supervise; result should be a structure. Managers supervise at least one employee.

Find serial numbers and streets of all ATMs belonging to branch "London Westend" which have at least one currency order that was approved by "Martin Holmes".

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Practice Solution:Define OQL Queries

Find all ATMs which can supply at least 1000 "Indian Rupees".

SELECT a FROM a IN ATM WHERE EXISTS c IN a.CurrencyBalances: c.Currency = 'Indian Rupees' AND c.Balance >= 1000

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Practice Solution:Define OQL Queries

Find all managers with the number of employees they supervise; result should be a structure.

SELECT STRUCT ( Manager: m, NrSubs: COUNT(m.Subordinates))FROM m IN EmployeesWHERE NrSubs > 0

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Practice Solution:Define OQL Queries

Find serial numbers and streets of all ATMs belonging to branch "London Westend" which have at least one currency order from Mr. "Hog".

SELECT STRUCT ( SerialNr: a.SerialNr, Street: a.Address.Street)FROM a IN ATM, c IN a.CurrencyOrdersWHERE c.Requestor = 'John Hog' AND

a.Branch = 'London Westend'

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4.8 Methods …

perform class specific operations are a major difference to pure relational model allow to combine structural and operational

development of an application. can be class methods or instance methods can be inherited from superclasses Polymorphism: Subclass overrides method

inherited from superclass

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Generic Update Methods

Operation Object oriented operation Operation in Caché

Insert Constructor %New() classmethod

%Save() instancem.

Update Access to attribute set obj.property = NewVal

Delete Persistent destructor %DeleteId(Id)classmethod

Insert Copy Copy %ConstructClone(d)

d: deep or shallow copy

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Where to Code User Specific Methods?In Database or in Client Application?

Code methods in database classes, if they are used by many different client applications access other persistent objects query database or perform database operations build a unity with the attributes

Code methods in client application, if they perform time consuming calculations need a large amount of main memory belong to the user interface are invoked concurrently by many clients to use parallel

processing on many client computers.

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Practice:Client or Server Side Method

Where would you code the following methods? Discuss with your neighbor. Find other examples for client side and for server side method. Sum up the amounts of available money for each

currency in all ATMs A currency exchange transaction is prepared at home.

Calculate the amount of money to be supplied for a certain requested currency value.

A CAD application stores data of the developed machines in a database. The engineer can view a drawing of the machine from different perspectives, turn it etc. Calculate the drawing.

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Methods in Caché

Code written in ObjectScript: allows use of embedded SQL and macros.

or in Caché Basic (similar to Microsoft VBScript). Set Language attribute of a class (for all methods)

or individual method(s). Can be mixed.

ObjectScript can call Basic. Basic can call ObjectScript.

Both languages compile to same OBJ code.

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Inherited Methods

Classes that extend other classes inherit their methods.

For example, new persistent classes inherit methods of %Persistent class.

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Method Signature C H ÉA C HC A C

Method MyMethod (Arg1 As %String = 'abc', Arg2 As %Numeric)As %String

{ set x = …}

method name

arguments

argument name

argument type

default value

return type:• result of calculation or• object or• status

body

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Method Arguments

Arguments are private to method. Caller is not required to supply all arguments to a

method:do ##class(Package.Class).Method(1,,3,,4)

Method must either provide default value for each argument, or use $data or $get to check argument.Method Test(a as %String = 1, b as %String) as %String

{if '$data(b) { code here }

}

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ObjectScript Arguments

Pass by value. Default. Use ByVal in Code Window to make explicit. Value passed in for input only.

Pass by reference. Optional. ByRef in Code Window required.

Inspector uses & before name. Reference passed in for input and/or output. Optionally, use Output in Code Window to document that

argument is for output only. Use * before name in Argument dialog.

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134

ObjectScript Arguments (cont.)

Pass by reference/value is for documentation purposes only. Shows user how to call method.

Caller, not signature, determines whether argument is passed by reference or by value. Period before argument specifies pass by reference.

For example: By value: do oref.Method(a,b) By reference: do oref.Method(.a,.b)

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Objects as Arguments

Object arguments may be OREFs. If method changes properties of object only, object

argument may be passed by value. Argument inside method references same object as

calling method. If method changes object argument to reference a

different object, argument must be passed by reference, in order to return new object to caller.

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Method Overloading

Caché doesn’t provide method overloading in a way similar to other languages such as Java. Can’t create multiple methods with the same name in the

same class. To simulate method overloading in Caché:

Use $data/$get to check arguments. Call different code depending on which arguments are

sent.

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Private Methods

Methods are public by default. Methods of other classes can use a public method.

Set Private attribute to True to make method private. Only methods in same class or subclasses can use a

private method.

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Class and Instance Methods

ClassMethod attribute controls whether method is class or instance method. True: Call without an object in memory. Use ##class

syntax. False: Call on a specific object in memory.

Examples:set branch = ##class(FCE.Branch).%New() set status = branch.%Save() // save THIS branch

ClassMethod=True and SQLProc=True: method projected as SQL stored procedure.

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Relative Dot Syntax

Inside an instance method, use relative dot syntax (..) to refer to another property or method of current instance. Read as “this object’s...” In Caché Basic, use Me.

Examples: ..Property ..Method()

Instance or class method. ..#ClassParameter

ObjectScript only.

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Accessor Methods

All properties have hidden “getter” and “setter” accessor methods. PropertyGet() and PropertySet() instance methods. Called automatically when code references property. Not visible in Studio.

Each pair of statements is equivalent:write oref.Property write oref.PropertyGet()

set oref.Property = value do oref.PropertySet(value)

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Overriding Accessor Methods

For special property processing when getting or setting properties, override accessor methods. Last panel of New Property wizard allows this.

Also used for Computed Properties. Override has no effect on SQL operations.

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Practice: Develop Method(s) to Process a Request

A person wants to change money. He/she provides data for the requested currency, requested amount, and supplied currency.These data are sent to the database application and must be processed there.

Discuss the following questions Which data must the ATM send to the server? In which class would you implement the method(s) to

process a request? What is/are the method's signatures?

Develop a concept for the method(s) (flow chart, sequence diagram, or pseudo code)

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SQL Stored Procedures

Stored procedure is nothing more than a class method that is made available to SQL

Stored procedure may be called from an external tool via ODBC/JDBC.

Class queries are stored procederes Class queries return result set only. They can't have side effects SqlProc=True causes query to be listed in catalog for

external tool. Method Stored Procedures do not return a set of

records may have side effects.

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SQL Stored Procedures (cont.)

To make class method available as stored procedure: Must be class method. SqlProc=True.

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Stored Procedure Code

Code can be ObjectScript (with or without embedded SQL) or Basic.

Method can return value and/or have input/output arguments (simple data, streams, or objects). Be careful with return value and argument types.

Method can also return result set (refer to Query Extent() for an example).

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%sqlcontext Object

Code should check %sqlcontext to determine whether or not method called as stored procedure.if $IsObject($get(%sqlcontext)) { code here }

Use properties of %sqlcontext to return information to external tool: SQLCode: SQLCODE error number, or 0 if no error. Message: error message text. RowCount: %ROWCOUNT.

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Calling a Stored Procedure

Returning result set:call Schema.Table_QueryOrMethodName(arguments)call Schema.Table_QueryOrMethodName arguments

Using return value as part of query. For example:select columnsfrom tableswhere column = Schema.Table_QueryOrMethodName(arguments)

Using ? as placeholder for return value and input/output arguments:? = Schema.Table_QueryOrMethodName(?, ?)

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Instance Methods

Instance query-methods do not have side effects provide a value of the object can be interpreted as additional property of the object In Caché use computed properties for convinient access

by SQL and object code Instance update-methods

change the status of the object test, whether an operation is allowed In Caché SQL access not possible, use stored

procedures instead, with object-id as argument.

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Computed Properties

Caché supports two types of computed properties/columns. Always-computed.

Property value computed whenever code references property. Triggered-computed.

Triggered-computed property/column is stored. Computation triggered when new object is created/inserted,

or existing object is changed/updated(properties listed in SqlComputeOnChange attribute).

Not triggered when object is opened, nor when row is selected. Both types compute property/column value based on

another property/column (or other properties/columns).

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SqlComputeCode Examples

Age calculated based on DOB and $horolog: set {Age}=$select(({DOB}'=""):((+$h-{DOB})\365.25),1:"" ) $select is a line based if- or case-statement This code means

if {DOB} '= "" {set {Age} = ($piece($horolog,",",1)-{DOB})\365.25}

else {set {Age}=""}

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SqlComputeCode Attribute

Specify computation using SqlComputeCode attribute. ObjectScript containing {ColumnName} reference(s).

SQL analog for ..PropertyName syntax. Unlike regular ObjectScript, no extra spaces allowed.

{ColumnName} is: Property’s SqlFieldName attribute, if present. Otherwise, property’s name.

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How To:Create a Computed Property

Create new property. Set SqlComputed attribute = True. Set Calculated attribute:

For always-computed, set = True. For triggered-computed, set = False.

For triggered-computed, optionally specify SqlComputeOnChange attribute: List of properties/columns that trigger computation.

Specify SqlComputeCode.

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4.9 Integrity

The object oriented model has inherent integrity constraints. Foreign key constraints are not necessary, due to

component and subclass relation NOT-NULL-Constraint

In Caché use the Required attribute for a property UNIQUE-Constraint:

A combination of attributes must be unique for each object.

In Caché use an index with the Unique attribute. An Index is also used for access optimization.

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Integrity (cont.)

CHECK-Constraint In Caché use parameters like PATTERN, VALUELIST,

MINVAL, or MAXVAL Example Phone: PATTERN = "3n1""-""3n1""-""4n"

DEFAULT-Constraint In Caché use InitialExpression attribute of a property

Use set- and get-methods to access private properties instead of public properties You can code difficult constraint-checks in these

methods.

155

Callback Methods and Triggers

Use Callback methods or triggers for sophisticated constraint checking or consistency operations.

Callback methods are used on object side. Triggers are used on SQL side. Triggers do not check object operations!

Callback methods do not check SQL operations! To unify the functionality for both OOP and SQL,

create class method that callback method and trigger both call.

!

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Callback Methods and Triggers

do ##class(X).%DeleteId(22) delete from X where %id = 22

Caché

class X

ClassMethod %DeleteId()

ClassMethod %OnDelete()

Trigger XBeforeDelete

SQL

X: 22, abc,

do ##class(Y).%DeleteId(5) delete from Y where %id = 5

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OOP Callback Methods

Events during lifetime of an object can “call back” to methods you supply.

Each callback method can check object and affect outcome of its event.

Naming convention: %OnEventName(). Object classes inherit callback method signatures.

Override with your code. To prevent operation (e.g. prevent saving an

object) return an error in the appropriate callback method (e.g. in %OnValidateObject()):quit $$$ERROR($$$GeneralError, "My error message")

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Saving Object

%Save() calls: Method %OnAddToSaveSet(depth,insert).

The save set is the set of objects to be saved. Argument: insert = 1 for a new object. Return error status to prevent save of object. Use this pre-validation callback for creating new objects,

modifying properties of existing objects, or adding objects to the save set.

Method %OnValidateObject(). Use this pre-validation callback for custom object validation. Return error status to cause object to fail validation.

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Saving Object (cont.)

%Save() calls: Method %OnBeforeSave(insert).

Use this post-validation callback for time-sensitive object changes, such as setting Creation/Edited timestamp properties.

Argument: insert = 1 for a new object. Return error status to prevent save of object.

Method %OnAfterSave(insert). Argument: insert = 1 for a new object. Return error status to rollback save of object.

Method %OnRollBack(). Called when rollback occurs. Return error status to prevent rollback.

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Deleting Object from Disk

%DeleteId() calls: ClassMethod %OnDelete().

Argument: OID (not ID) of object to be deleted. Use $$$oidPrimary macro to get ID from OID.

Return error status to prevent deletion.

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Other Callback Methods

Method calls Callback Method Comment

%New() %OnNew() Good place for constructor code

%OpenId() %OnDetermineClass()

%OnOpen()

Class methodInstance method

%ConstructClone() %OnConstructClone(object, deep)

object is OREF of cloned object

deep is deep argument of %ConstructClone()

set oref = "" kill oref

%OnClose() Removing Object from Memory

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Practice: Write Callback Method

A branch must not close before it opens. Discuss different possibilities to enforce

that constraint. Write a callback method that enforces the

constraint.

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Practice Solution: Write Callback Method

A branch must not close before it opens. Discuss different possibilities to enforce that

constraint. Set MINVAL parameter for Closes greater than

MAXVAL parameter for Opens Instead of Closes property Branch gets OpenTime

property with MINVAL > 0; implement Closes as computed property Closes = Opens + OpenTime

Callback method %OnValidateObject() and Trigger BranchCheckOpensCloses

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Practice Solution: Write Callback Method

A branch must not close before it opens. Write a callback method that enforces the

constraint.

Method %OnValidateObject() As %Status[Private]{ if (..Closes < ..Opens) { quit $$$ERROR($$$GeneralError, "Branch closes before it opens")

} else {Quit $$$OK}}

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SQL Triggers

Events during lifetime of a row can “trigger” methods you supply: Events: Insert, Update, Delete by SQL

Each trigger can check current row and affect outcome of its event. Can also issue additional SQL statements (which may

trigger additional methods). Trigger code called after validation and constraint

checking.

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SQL Triggers (cont.)

Trigger events: Insert, Update, Delete. Trigger timings: Before, After. For multiple combinations of the same event and

timing, specify an Order attribute. For example:Trigger T1 [Event = INSERT, Time = AFTER, Order = 1]

{ code here }Trigger T2 [Event = INSERT, Time = AFTER, Order = 2]

{ code here }Trigger T3 [Event = UPDATE, Time = BEFORE]{ code here }

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Row-Based Triggers

Caché triggers are row-based rather than statement-based.

For example, an update of 20 rows will trigger an after update trigger for each row.

Some other relational products’ triggers would trigger after update trigger once after updating all rows.

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Writing a Trigger

Trigger code can use embedded SQL and {ColumnName} references to access value of any column in current row. Triggers can’t change value of columns in current row.

For UPDATE triggers, code can use: {column*o} to access old value. {column*n} to access new value. {column*c} true if data changed.

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Preventing Event

To prevent event from occurring:set %ok = 0, %msg="ErrorMessageText"

Used in before triggers, event changes never happen.

Used in after triggers, causes rollback of changes.

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Practice: Write Trigger

A branch must not close before it opens. Write a trigger that enforces the

constraint.

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PracticeSolution: Write Trigger

A branch must not close before it opens. Write a trigger that enforces the constraint.create trigger FCE.BranchCheckOpenTimebefore insert on FCE.Branchlanguage OBJECTSCRIPT{if ({Closes*N} < {Opens*N}) { set %ok = 0 set %msg = "Branch closes before it opens" }}

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Modern Database TechniquesPart 1: Object Oriented Databases

5. Application Development on Client Side

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5. Client Application Development

Self study project for students: Develop a client application for the FCE example,

simulating an ATM. Java application or web application. Use Caché language binding techniques. Tutorial with detailed instructions available. Starter files for GUI available. Solution available.