Entity Framework Code First: One-to-Many and Many-to-Many relationships to same table

Question

I have a User model and a Event model in my project. The Event has a creator(User) and has participant(Users) so Event has a one-to-many relationship with User and also a many-to-many relationship to the same table.

I had first the one-to-many relationship like this:

Public class Event
{
      ...
      public int CreatedById { get; set; }
      public virtual User CreatedBy { get; set; }
      ...
}

Then when I added the many-to-many relationship the migration doesn't generate the many to many relationship:

Public class User
{
      ...
      public virtual ICollection<Event> Events { get; set; }
      ...
}

Public class Event
{
      ...
      public int CreatedById { get; set; }
      public virtual User CreatedBy { get; set; }
      public virtual ICollection<User> Users { get; set; }
      ...
}

If I remove the one-to-many relationship then the migration generates the many-to-many relationship successfully.

Is there a way to do this with only data annotations?

Answer 1

EF doesn't know where User.Events has to be mapped to. It could be Event.CreatedBy or it could be Event.Users. Both would result in a valid model. You must give EF a little hint what you want by applying the [InverseProperty] attribute:

public class User
{
    ...
    [InverseProperty("Users")]
    public virtual ICollection<Event> Events { get; set; }
    ...
}

Answer 2

With Code First Approach, I would always recommend to use fluent API rather than using DataAnnotations, Which uses some conversions automatically.

This way, you'll know what exact configuration you've made.

If I were you, here is what i would use :

public class EventMap : EntityTypeConfiguration<Event>
{
    public EventMap()
    {
        this.HasRequired(m => m.CreatedBy) // envent must have a creator
            .WithMany() // a user can have 0,1 or more events created by him
            .HasForeignKey(m => m.CreatedById) // specify property to be used as FK
            .WillCascadeOnDelete(true); // delete all events created by user if that specific user is deleted

        this.HasMany(m=>m.Users) // an event can have 0,1 or more participants
            .WithMany(m=>m.Events) // a user can be a participant in 0,1 or more events                
            .Map(m => m.MapLeftKey("EventId").MapRightKey("UserId")); // this will generate intermediate table to hold participant information - dbo.EventUser with EventId & UserId
            // Cascade Delete is always true for Many to Many mapping. however, it doesn't delete entry in other table, it deletes entry in Joined Table only.
    }
}