Entity Framework Many-to-many relationships, with the same link entity - entity-framework

Question

I am trying to get some reuse out of a class which holds the many-to-many relationship data between a number of entities.
You can do this easily enough where the relationship is simply the pks of the two entities using Map
class A{
icollection<Item> Items
}
class B{
icollection<Item> Items
}
class Item{
string Text
}
then in the config you do something like this
Entity<A>().HasMany(e=>e.Items).WithMany().Map(..);
Entity<B>().HasMany(e=>e.Items).WithMany().Map(..);
this produces a new link table for A and B
Now, I would like to store more info on the link tables, but this would be common
class A{
icollection<LinkItem> Items
}
class B{
icollection<LinkItem> Items
}
class LinkItem{
int ExtraInfo
Item Text
}
class Item{
string Text
}
This fails as the LinkItem is created in a single table and complains about relationships
"Entities in 'DataContext.LinkItem' participate in the 'A_Items' relationship. 0 related 'A_Items_Source' were found. 1 'A_Items_Keywords_Source' is expected."
You can specify a table in the Map method, m.ToTable("AItems"), but this fails too
"The specified table 'AItems' was not found in the model. Ensure that the table name has been correctly specified."
Am I allowed to reuse my LinkItem class somehow?
(It doesn't need to be accessed as a set from the DataContext)

Answer 1

I believe what you want can be done, I've just tried using EF5, with the following model.
public class UsersContext : DbContext
{
public DbSet<A> As { get; set; }
public DbSet<B> Bs { get; set; }
public DbSet<LinkItem> LinkItems { get; set; }
public DbSet<Item> Items { get; set; }
}
public class A{
public virtual int Id { get; set; }
public virtual ICollection<LinkItem> Items { get; set; }
}
public class B{
public virtual int Id { get; set; }
public virtual ICollection<LinkItem> Items { get; set; }
}
public class LinkItem{
public virtual int Id { get; set; }
public virtual DateTime ExtraInfo { get; set; }
public virtual Item Text { get; set; }
}
public class Item{
public virtual int Id { get; set; }
public virtual string Text { get; set; }
}
This resulted in the following DB model
I've not added any custom mappings just used what is inferred by the model. Does that work for you?
If you want to avoid the proliferation of the foreign keys on the LinkItems table, you can make it a many-2-many relationship with entities A and B with the following DbModelBuilder commands which will add the implicit link tables.
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<A>()
.HasMany(t => t.Items)
.WithMany();
modelBuilder.Entity<B>()
.HasMany(t => t.Items)
.WithMany();
}