EF代码优先1：相同模型之间的1：* 1：0..1关系

Question

I have two models

public class A{
    public AID { get; set; }

    public int? MainBID { get; set; }
    [ForeignKey("MainBID")]
    public B MainB { get; set; }
}
public class B{
    public int BID { get; set; }

    public int? AID { get; set; }
    [ForeignKey("AID")]
    public virtual A Owner { get; set;}
}

The relationship I want to model is A has many B. A may also have a specially designated B (main B).

However the way I have it set up it has a hard time identifying the principal and using fluent:

modelBuilder
    .Entity<A>()
    .HasOptional(x => x.MainB)
    .WithOptionalDependent();

It gives me Multiplicity is not valid in Role 'A_MainB_Source' in relationship 'A_MainB'. Because the Dependent Role properties are not the key properties, the upper bound of the multiplicity of the Dependent Role must be '*'.

I think my trouble is trying to describe to EF that these are two separate relationships and not the same relationship described front-and-back. Can anyone guide me in this?

EDIT: Adds an real life example

I have need to model a series of teaching courses (each is called a "Module"). All "Modules" represent a webpage conveying the information for that particular course. Each Module can have many "Resource"s (downloadable binary files).

Some modules can have a Resource that represents the same information as the webpage. As in some modules can have a Resource that can be printed out and consumed instead of sitting in front of your machine reading through the webpage. A PDF version of the webpage.

So in the above each module has many resources. But each module can optionally have a "special" resource that needs to be called out separately as well as wherever other resources are used.

I considered just having a flag on Resource to indicate that a resource is special in some way but that will allow for 0..* which is not what I want. I thought this might be cleaner. But I have so far not been able to get this to work with EF.

Answer 1

this is a working example:

public class Module
    {
       public int Id { get; set; }
       public string Name { get; set; }
       public ICollection<Resource> Resources { get; set; }
       public int? SpecialResourceId { get; set; }
       public Resource SpecialResource { get; set; }
    }

    public class Resource
    {
        public int Id { get; set; }
        public int? ModuleId { get; set; }
        public Module Module { get; set; }
        public virtual Module IsSpecialForModule { get; set; }
    }

    public class MyContext : DbContext
    {
        public virtual DbSet<Module> Modules { get; set; }
        public virtual DbSet<Resource> Resources { get; set; }


        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
           modelBuilder.Entity<Module>()
                .HasMany(x=>x.Resources)
                .WithOptional(x=>x.Module)
                .HasForeignKey(x=>x.ModuleId)
                .WillCascadeOnDelete(false);

            modelBuilder.Entity<Resource>()
                .HasOptional(x=>x.IsSpecialForModule)
                .WithOptionalDependent(x=>x.SpecialResource)
                .WillCascadeOnDelete(false);
        }
    }

if your special resource, can be special for many modules then your model is changing like this:

 public class Resource
    {
        public int Id { get; set; }
        public int? ModuleId { get; set; }
        public Module Module { get; set; }
        public virtual ICollection<Module> IsSpecialForModules { get; set; }
    }

and the config. will be :

   modelBuilder.Entity<Resources>()
                .HasMany(x=>x.IsSpecialForModules )
                .WithOptional(x=>x.SpecialResource )
                .HasForeignKey(x=>x.SpecialResourceId )
                .WillCascadeOnDelete(false);