Entity Framework Mapping relationship with Entity Framework Code First: One-to-one and variations Mapping one-to-one

Example

Mapping one-to-one (when both sides are required) is also a tricky thing.

Let's imagine how this could be represented with foreign keys. Again, a CarId in People that refers to CarId in Car, and a PersonId in Car that refers to the PersonId in People.

Now what happens if you want to insert a car record? In order for this to succeed, there must be a PersonId specified in this car record, because it is required. And for this PersonId to be valid, the corresponding record in People must exist. OK, so let's go ahead and insert the person record. But for this to succeed, a valid CarId must be in the person record — but that car is not inserted yet! It cannot be, because we have to insert the referred person record first. But we cannot insert the referred person record, because it refers back to the car record, so that must be inserted first (foreign key-ception :) ).

So this cannot be represented the 'logical' way either. Again, you have to drop one of the foreign keys. Which one you drop is up to you. The side that is left with a foreign key is called the 'dependent', the side that is left without a foreign key is called the 'principal'. And again, to ensure the uniqueness in the dependent, the PK has to be the FK, so adding an FK column and importing that to your model is not supported.

So here's the configuration:

public class CarEntityTypeConfiguration : EntityTypeConfiguration<Car>
{
  public CarEntityTypeConfiguration()
  {
    this.HasRequired(c => c.Person).WithRequiredDependent(p => p.Car);
    this.HasKey(c => c.PersonId);
  }
}

By now you really should have gotten the logic of it :) Just remember that you can choose the other side as well, just be careful to use the Dependent/Principal versions of WithRequired (and you still have to configure the PK in Car).

public class PersonEntityTypeConfiguration : EntityTypeConfiguration<Person>
{
  public PersonEntityTypeConfiguration()
  {
    this.HasRequired(p => p.Car).WithRequiredPrincipal(c => c.Person);
  }
}

If you check the DB schema, you'll find that it's exactly the same as it was in the case of the one-to-one or zero solution. That's because again, this is not enforced by the schema, but by EF itself. So again, be careful :)