EntityFramework foreign key as primary key with fluent API

Question

I'm trying to create a required:optional relationship between two entities, with the required entity exposing a navigation property to the optional entity, and the optional entity containing the foreign key, used as its primary key. This is what my two entities look like:

class OptionalEntity
{
  public string RequiredEntityID { get; set; }
}

class RequiredEntity
{
  public string ID { get; set; }
  public OptionalEntity Optional { get; set; }
}

And the way I would like to configure them in fluent API is as follows:

// Inside OptionalEntityConfiguration class
public OptionalEntityConfiguration()
{
  HasKey(r => r.RequiredEntityID);
}

// Inside RequiredEntityConfiguration class
public RequiredEntityConfiguration()
{
  HasKey(r => r.ID);
  HasOptional(r => r.Optional)
    .WithRequired();

  //  How can I configure this relationship to use
  //    the RequiredEntityID property as the foreign key?

  HasOptional(r => r.Optional)
    .WithRequired(o => o.RequiredEntityID);
  //  This is invalid because it requires a navigation property, not an ID

  HasOptional(r => r.Optional)
    .WithRequired()
    .HasForeignKey(o => o.RequiredEntityID);
  //  The HasForeignKey method isn't available here
}

First of all is this possible, and if so what's the correct way to configure this relationship using fluent API?

HasOptional(r => r.Optional).WithRequired(); is enough. EF knows that OptionalEntity has RequiredEntityID as PK and its standard way of implementing 1-0..1 is by making this a PK/FK. — Gert Arnold

Frank Fajardo Frank Fajardo · Accepted Answer · 2016-02-02T22:45:19

I think what you are trying to do is use the same key used on the Required Entity on the Optional Entity table, so that they share the same key.

If that is the case, I think you are on the right track. Your entity classes look alright. You can map them like these:

public OptionalEntityConfiguration()
{
   HasKey(r => r.RequiredEntityID);
   Property(r => r.RequiredEntityID)
      .HasDatabaseGeneratedOption(DatabaseGeneratedOption.None);
}

public RequiredEntityConfiguration()
{
   HasKey(r => r.ID);
   HasOptional(r => r.Optional);
}

This implies, however, that there can only be 0 or 1 OptionalEntity for each RequiredEntity.

EntityFramework foreign key as primary key with fluent API

2 Answers