1 active records. 2 what’s active records? o-r mapping layer to make database access almost a...
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2Venkat Subramaniam – [email protected] Subramaniam – [email protected]
What’s Active Records?• O-R Mapping layer
• To make database access almost a non-issue• Relies heavily on convention over configuration
– some say this is simple, others say this is simplistic– Tables map to classes– Rows map to objects– Columns map to attributes
• You write minimal code– Quite a bit of code is synthesized behind the scene
3Venkat Subramaniam – [email protected] Subramaniam – [email protected]
Accessing Data• You derive your class from
ActiveRecord::Base• That’s almost all you need• You are provided with methods you can
readily use• Attributes are inferred based on column
names in schema– You may have additional attributes
• Like having a clear text password in memory while the password column is encrypted
• Has primary key id by convention
4Venkat Subramaniam – [email protected] Subramaniam – [email protected]
Locking• Supports Pessimistic or Optimistic
Locking
• Optimistic if your table has an integer column named lock_version– Active Records take care of rest
• You can control this by ActiveRecord::Base.lock_optimistically = false
5Venkat Subramaniam – [email protected] Subramaniam – [email protected]
Classes to Tables• Active Record assumes the table name is
plural form of your model class– Multiword class name transorms to words
separated by underscores– Controlled by global glag in environment.rb
• ActiveRecord::Base.pluralize_table_names = false
– Common plurans and then some weird ones• people (Person), journals (Journal), children
(Child), dear_friends (DearFriend)
• You can break away from the convention (for legacy database) using set_table directive
6Venkat Subramaniam – [email protected] Subramaniam – [email protected]
MySql• Create mysql database
– mysql– create database csalum_development;
• Create a create.sql script for creating table
• Run script– mysql –p –r root csalum_development < create.sql
8Venkat Subramaniam – [email protected] Subramaniam – [email protected]
Exploring Active Records• Columns() method tells us
9Venkat Subramaniam – [email protected] Subramaniam – [email protected]
SQL Type to Ruby Type Mapping• Standard SQL to Ruby type mapping• int, integer -> Fixnum• decimal, numeric, float, double -> Float• interval, date -> Date• clob, blob, text, char, varchar, string -> String• datetime, time -> Time• Money? – decimal to float may lead to rounding
errors– For currency you may use
• units of cents• aggregate Money objects
10Venkat Subramaniam – [email protected] Subramaniam – [email protected]
Accessing Attributes• Active Records converts column values to
appropriate Ruby types• You can get raw value of an attribute as well by
appending attribute name with _before_type_cast
• Use caution with boolean types– No consistent represenation in databases– Use the ? form of method to get correct value– Still poses problems for i18n
11Venkat Subramaniam – [email protected] Subramaniam – [email protected]
CRUD• Active Records is based on the simple
notion that you mostly need basic operations on tables– Create, Read, Update, and Delete
• Methods for these are synthesized behind the scene– new– finders– save– update– Delete
12Venkat Subramaniam – [email protected] Subramaniam – [email protected]
Creating A Row• Use new to create object• Remember to save
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create() Method• Combines new and save method• create(hash) => object• create(array_of_hash) => objects
15Venkat Subramaniam – [email protected] Subramaniam – [email protected]
Reading• find takes primary key or an array of
primary keys• Gets fancier than that as well
– Can take other parameters
• :first specified to return first matching row
• :all specified to return all matching rows• :condition helps send parameters to SQL
where clause
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Reading
No guarantee on ordering unless you specify order by
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Specifying Criteria• Unsafe way – SQL Injection problem
– Don’t try this at home
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Specifying Criteria – Better Ways
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Specialized finds• You can search based on column values• Methods synthesized for you
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Update/Save• Save updates existing row or inserts a new row• save returns true if model is valid and can be
saved• save! raises a RecordInvalid exception if objects
can’t be saved• You may also use update_attribute or
update_attributes• Class method update allows you to update a
row without reading it• update_all is like SQL update with SET and
WHERE clause
21Venkat Subramaniam – [email protected] Subramaniam – [email protected]
Deleting• destroy allows you to delete a row based
on id or condition
• Once deleted, object in memory is frozen
• destroy_all is a classes methods that destroys all objects that meet given condition