实体框架 - 多对多？

Question

I'm defining a many-to-many relationship as follows:

    modelBuilder.Entity<GameSessionEntry>().
         HasMany(c => c.Users).
         WithMany(p => p.GameSessionEntries).
         Map(
          m =>
          {
              m.MapLeftKey("SessionId");
              m.MapRightKey("UserId");
              m.ToTable("UserSessions");
          });

However, I keep getting:

The Foreign Key on table 'UserSessions' with columns 'UserId' could not be created because the principal key columns could not be determined. Use the AddForeignKey fluent API to fully specify the Foreign Key.

I'm new to database work and the EntityFramework in general - what is it asking me to do?

Answer 1

It's the recurring confusion with left and right, see this explanation by Slauma. So you just have to turn around the key names:

  m.MapLeftKey("UserId");      // Property in the HasMany call
  m.MapRightKey("SessionId");  // Property in the WithMany call

Answer 2

This is how I usually go about creating a many to many table (note this requires no fluent api configuration)

public class User
{
    public int Id { get; set; }
    public virtual ICollection<UserSession> UserSessions { get; set; }
}

public class Session
{
    public int Id { get; set; }
    public virtual ICollection<UserSession> UserSessions { get; set; }
}

public class UserSession
{
    [Key]
    [Column(Order = 1)]
    public int UserId { get; set; }

    [Key]
    [Column(Order = 2)]
    public int SessionId{ get; set; }

    public virtual User User { get; set; }
    public virtual Session Session { get; set; }
}

Answer 3

Instead of fiddling around with a many-many relationship you should rewrite it to a weak entity set.

If you have for instance this relationship:

You can redesign it to a weak entity set:

By doing this you get rid of the many-many relationship and don't have to store the same data in multiple tables.

For more information: http://fileadmin.cs.lth.se/cs/Education/EDA216/lectures/dbtoh4.pdf Read the lecture slides about "The Relational Data Model" starting on slide 87/360.