How to make One-To-One relationship with existing fields?

Question

I have 3 entities:

• Foo
• Bar
• UniqueFooBar

Foo and Bar are entities as follows:

public class Bar {

   public int Id {get; set;}

   // inverse nav property
   public virtual UniqueFooBar UniqueFooBar {get; set;}

}

public class Foo {

   public string Name {get; set;}

   // inverse nav property
   public virtual UniqueFooBar UniqueFooBar {get; set;}
}

And UniqueFooBar is a lookup as follows:

public class UniqueFooBar {

   public string FooName {get; set;}
   public int BarId {get; set;}

   // nav properties
   public virtual Foo Foo {get; set;}
   public virtual Bar Bar {get; set;}

}

With constraints that:

• Foo is unique
• there is a one-to-one relationship to both Foo and Bar

• Foo Name is the PK

  The fluent API is as follows:

  class UniqueFooBarConfiguration : EntityTypeConfiguration<UniqueFooBar> {
      public UniqueFooBarConfiguration() {
          // Define the tablename and schema
          Map(entity => entity.ToTable("UniqueFooBars"));
  
          //// Define non-conventional key
          HasKey(fooBar => fooBar.FooName);
  
          // Define FKs -  1-to-1
          HasRequired(fooBar => fooBar.Foo)
              .WithRequiredPrincipal(foo => foo.UniqueFooBar)
              .Map(key => key.MapKey("FooName"));
          HasRequired(fooBar => fooBar.Bar)
              .WithRequiredPrincipal(bar => bar.UniqueFooBar)
              .Map(key => key.MapKey("BarId"));
          // --------------------------------
  
      }
  
  }
  

What is happening is that FooName is being added to the Foo table and BarId is being added tot he Bar table.

If, in the fluent API configuration for UniqueFooBar, I instead try to use Foo's "Name" property then there is an error that the field already exists. Same happens if I try to use Bar's "Id" property.

How can I configure UniqueFooBar to have FKs to Foo.Name and Bar.Id as one-to-one relationships?

Update

• Neither Foo nor Bar has constraint or requirement of a UniqueFooBar.
• A UniqueFooBar record requires a FooName and a BarId

This does not appear to be the same as How to declare one to one relationship using Entity Framework 4 Code First (POCO)

Answer 1

Taken from here, below is an example of how you can achieve one to one mapping between two entities, extrapolate this for a link table, adding HasRequired as required.

It is possible to specify WithRequiredPrincipal without a lambda, which allows you to exclude a navigation property and still get a proper one to one mapping.

Within an override of the OnModelCreating method you define your relationships by using the DBModelBuilder parameter.

public class Customer
{
    public Customer()
    {
        Address = new Address();
    }

    public Guid Id { get; set; }
    public string Name { get; set; }
    public Address Address { get; set; }
}

public class Address
{
    public Guid Id { get; set; }
    public string City { get; set; }
    public string Country { get; set; }
    public string Street { get; set; }
}

public  class  CustomerContext : DbContext
{
    public IDbSet<Customer> Customers { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
         modelBuilder.Entity<Customer>()
                .HasRequired(x => x.Address)
                .WithRequiredPrincipal();
         base.OnModelCreating(modelBuilder);
    }
}