Unidirectional One-To-One relationship in Entity Framework

Question

Below example of one-to-one relationship throws an exception

public class User
{
    public int Id { get; set; }
    public Address Address { get; set; }
}

         


        

Answer 1

While the answer provided by Eranga is correct and creates a shared primary key association between User and Address, you might not want to use it due to the limitations this mapping type has.

Here is another way of creating a 1:1 association which is called one-to-one foreign key association:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<Address>()
                .HasRequired(a => a.User)
                .WithOptional(u => u.Address)
                .Map(m => m.MapKey("UserId"));
}

EF Code First recognizes this as a 1:1 association hence allowing you to have a bidirectional association between User and Address.

Now all you need to do is to define a Unique Key constraint on the UserId column to make your relationship a true one to one on your database side. One way for doing so is using a Seed method that has been overridden in a custom initializer class:

class DbInitializer : DropCreateDatabaseAlways<Context>
{
    protected override void Seed(Context context)
    {
        context.Database.ExecuteSqlCommand("ALTER TABLE Addresses ADD CONSTRAINT uc_User UNIQUE(UserId)");
    }
}

The above code will result in the following schema:

Answer 2

You need to use fluent API to map the relationship as shared primary key.

public class MyContext : DbContext
{
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
         modelBuilder.Entity<Address>()
             .HasRequired(a => a.User)
             .WithOptional(u => u.Address);
    }
}