Question

I have a sample code:

public class Tag
{
    public int TagId { get; set; }
    public virtual ICollection<User> Users { get; set; }
}

public class User
{
    public int UserId { get; set; }
    public virtual ICollection<Tag> Tags { get; set; }
}

When I run EF over my model (i use code first approach), i get some tables automatically created in my db:

Users
Tags
UserTagUsers <-- junction table for many-to-many relationship

It is okay, till I decide to add one more property to User entity:

public class User
{
    public int UserId { get; set; }
    public virtual ICollection<Tag> Tags { get; set; }
    public virtual ICollection<Tag> Tags2 { get; set; }
}

in this case EF generates completely different relations, it removes UserTagUsers junction table, but adds some additional properties to Tags table in order to make it one-to-one mapping.

How can I explicitly tell EF to make the property Tags and Tags2 to be many-to-many?

Answer 1

Use fluent API to configure the mappings

    modelBuilder.Entity<User>()
    .HasMany(u => u.Tags).WithMany(t => t.Users)
    .Map(m =>
    {
        m.ToTable("UserTags");
        m.MapLeftKey("UserId");
        m.MapRightKey("TagId");
    });

    modelBuilder.Entity<User>()
    .HasMany(u => u.Tags2).WithMany(t => t.Users2)
    .Map(m =>
    {
        m.ToTable("UserTags2");
        m.MapLeftKey("UserId");
        m.MapRightKey("TagId");
    });