Question

I need to map a many-to-many relationship using Entity Framework Code First. Its a standard socialnetworking FriendRequests mapping. A User Object has a Collection of FriendRequests of type List<User>. In the database I’m using a join table for the relationship as follows:

CREATE TABLE dbo.FriendRequests(
  UserId INT NOT NULL FOREIGN KEY REFERENCES dbo.Users(id),
  FriendId INT NOT NULL FOREIGN KEY REFERENCES dbo.Users(id),
  RequestDate SMALLDATETIME NOT NULL DEFAULT GETUTCDATE())
GO
ALTER TABLE dbo.FriendRequests ADD PRIMARY KEY (UserId,FriendId) 
GO

How do I map the user object in Entity Framework Code First to enable a Collection via a join table?

Answer 1

You can try it this way:

public class User
{
    public int Id { get; set; }

    public ICollection<FriendRequest> FriendRequests { get; set; }
}

public class FriendRequest
{
    public int UserId { get; set; }
    public int FriendId { get; set; }

    public DateTime RequestDate { get; set; }

    public User User { get; set; }
    public User Friend { get; set; }
}

Mapping with Fluent API:

modelBuilder.Entity<User>()
    .HasMany(u => u.FriendRequests)
    .WithRequired(f => f.User)
    .HasForeignKey(f => f.UserId);

modelBuilder.Entity<FriendRequest>()
    .HasKey(f => new { f.UserId, f.FriendId });

modelBuilder.Entity<FriendRequest>()
    .HasRequired(f => f.Friend)
    .WithMany()
    .HasForeignKey(f => f.FriendId);

Because of the RequestDate property in the link table you cannot map this as a many-to-many relationship. Instead you need two one-to-many relationships as shown above.

Possibly you need to disable cascading delete, I am not sure. You can do this by appending .WillCascadeOnDelete(false) at the end of the two one-to-many mappings.

Edit

To your comment: If you remove the RequestDate column you can create your model with a many-to-many relationship. You don’t need the FriendRequest entity in this case:

public class User
{
    public int Id { get; set; }

    public ICollection<User> FriendRequests { get; set; }
}

Mapping with Fluent API:

modelBuilder.Entity<User>()
    .HasMany(u => u.FriendRequests)
    .WithMany()
    .Map(m =>
    {
        m.ToTable("FriendRequests");
        m.MapLeftKey("UserId");
        m.MapRightKey("FriendId");
    });