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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

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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

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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

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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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