Entity Framework Code First One to One relationship on composite key

Question

Basically what I am trying to achieve is I have three tables, one parent will always have an entry in it and only one of the other two tables will be populated. The complexity that I am dealing with is that the primary key for the tables is the combination of two fields

ParentTable
-----------
UniqueID
OwnerID
[Some more fields]


ChildTable1
-----------
UniqueID
OwnerID
[Some more fields]


ChildTable2
-----------
UniqueID
OwnerID
[Some more fields]

I was wondering if anyone has any suggestions on how best to do this through EF Code First preferably using the Fluent API.

Answer 1

You just need to define that the primary keys are composite...

modelBuilder.Entity<Parent>().HasKey(p => new { p.UniqueID, p.OwnerID });
modelBuilder.Entity<Child1>().HasKey(c => new { c.UniqueID, c.OwnerID });
modelBuilder.Entity<Child2>().HasKey(c => new { c.UniqueID, c.OwnerID });

...and then define the two one-to-one relationships:

modelBuilder.Entity<Parent>()
    .HasOptional(p => p.Child1)
    .WithRequired(); // or .WithRequired(c => c.Parent)

modelBuilder.Entity<Parent>()
    .HasOptional(p => p.Child2)
    .WithRequired(); // or .WithRequired(c => c.Parent)

You cannot define a constraint though (except probably by defining a trigger in the database) that would ensure that a given parent may only refer to one of the two children, but not to both. You must handle this restriction in your application's business logic.