03 managing relationships

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03 | Managing Relationships

Adam Tuliper | Technical EvangelistChristopher Harrison | Content Developer

Managing Relationships• One to Many Relationships• One to One Relationships• Many to Many Relationships


One to Many Relationships

What have we seen so far?• "It just works" –Jon Galloway• Basic steps– Create a class– Add properties– Decorate with attributes as needed

Class design

Album

TitleReleaseYear

Artist

NameBio

1

*

So, if I wanted to create a relationship…• I should be able to just add properties, right?

DEMOLet's just see what happens…

Well, that didn't work!

Why did we get a null reference error?• Entity Framework uses lazy loading by default• "Magic" to make this work uses dependency injection

& inheritance

Lazy loading in actionAlbum

Title : stringReleaseYear : int

Artist : Artist

1. When Album is loaded, all of the simple data types are retrieved

2. A stub is placed into all of the complex data types• This stub contains the logic to have its

data loaded when it's first used3. When a property or method is called from

the complex type, the request is then made to load the data

Album album = context.Find(42);Console.WriteLine(album.Title);

1. Console.WriteLine(album.Artist.Name);

How do we support lazy loading?• Mark all complex type properties as virtual

• Can I disable lazy loading?– Short answer: yes– Longer answer: need to tell EF what to load and when to

load it• Adam will talk about this later today

Is that all I need to know?• Well… Not really…

• When loaded the child property is null• If you save the object to the database, it will attempt

to save the object with a null property– This will raise a referential integrity error

How do I solve that?• Add a property for the key of the primary object– ArtistID for Artist

• Entity Framework will automatically pick this up based on convention– Use the ForeignKeyAttribute if you need to change the

name

DEMOOne-to-many relationships, for real

Final one-to-many relationship note• Cascade delete is set to true– Deleting an Artist deletes their albums– Can be changed with the Fluent API


One to one relationships

One-to-one relationships aren't common• …and Entity Framework knows it• As a result, it's not expecting it– The default is one-to-many–When two classes point to one another, EF can't determine

which is the parent and which is the child• One-to-one relationships must be explicitly created– Add ForeignKey to the child class

DEMOOne-to-one relationships


Many-to-many relationships

Many-to-many relationship concepts• Relational databases typically don't support many-to-

many relationships natively– Requires a "join table" be created

• Fortunately, EF knows this– Just add the properties to both sides– EF will create the join table

DEMOMany-to-many relationships

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03 managing relationships

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