CODEWITHSUNDEEP
c#entity-framework
user2864862
user2864862

Reputation: 1

Entity Framework Code First Relationships; what am I missing?

I'm experimenting with EF5 Code First and I am using the models (show below).

When I look at the database that is created, I am confused because I do not see anything in the Track table that points to the Category table. Category has a FK pointing back to Track but that means that there are going to be duplicates of the categories?

A little background: I am trying to build a model that has tracks and every track can have 1 to N Categories. All of the categories are already defined, that is they are basically a lookup and I plan to create them in the seed method when database is created.

I think I am not understanding something obvious... When I query a track, how will I know what category it contains?

Thx

public class Track : IAuditInfo
{
    public Int32 Id { get; set; }
    public String Name { get; set; }
    public String Description { get; set; }
    public String Data { get; set; }
    public DateTime CreatedOn { get; set; }
    public DateTime ModifiedOn { get; set; }

    public ICollection<Category> Categories { get; set; }

    public Track()
    {
        Categories = new List<Category>();
    }
}

public class Category
{
    public Int32 Id { get; set; }
    public Boolean IsVisible { get; set; }
    public String DisplayName { get; set; }
}

Upvotes: 0

Views: 257

Answers (2)

flytzen
flytzen

Reputation: 7438

Olav is right, your data model at the moment is not telling Entity Framework that there is a many-to-many relationship in there.

The simplest way to resolve this is to add 

public virtual ICollection<Track> Tracks { get; set; }

to your Category class.

However... You may not want to pollute your domain model with artefacts that are not relevant to your domain. More importantly, when you do it this way, it is up to Entity Framework to figure out what to call the binding table. Prior to EF6 this naming is non deterministic (see http://entityframework.codeplex.com/workitem/1677), which may mean that two different machines compiling the same code will decide on different names for that table and cause some interesting migration problems in your production system.

The answer to both problems is to always explicitly manage many-to-many relationships with Fluent Configuration.

In your Data Context class, override the OnModelCreating, something like this:

public class MyDb : DbContext
{
    public IDbSet<Track> Tracks { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Track>()
           .HasMany(t => t.Categories)
           .WithMany()
           .Map(c => c.ToTable("CategoriesForTracks"));
    }
}

If you do this, you don't need to add a navigation property to your Category class, though you still can (if you do, you should use the overload for WithMany that allows you to specify a property).

Relationships between entities and how to map that to a relational database is inherently hard. For anything other than the simplest parent-child relationships you will want to use the fluent API to make sure you actually get what you want.

Morteza Manavi has a really good blog series describing relationships in EF Code First in exhaustive detail.

NOTE You should usually make navigation properties virtual. So, you should change your Category class like this:

public virtual ICollection<Category> Categories { get; set; }

In theory, not making it virtual should just cause eager loading rather than lazy loading to happen. In practice I have always found lots of subtle bugs appearing when my navigation properties are not virtual.

Upvotes: 0

Olav Nyb&#248;
Olav Nyb&#248;

Reputation: 11568

Your current model is a one-to-many relationship between tracks and categories. This usually implemented, as you have noted that entity framework does, using a foreign key on the many side (category) to the one side (track).

If I understand you correctly, what you want is a many-to-many relationship. Many tracks can be related to the same category, and a single track can belong to many categories.

To let entity framework understand that you want a many-to-many relationship you can simply add a ICollection property to your category class. So both your classes should have a collection of the other class. I.e. tracks have many categories and categories have many tracks.

For more information you can also see: http://msdn.microsoft.com/en-us/data/hh134698.a.nospx

Upvotes: 1

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