One to one optional relationship using Entity Framework Fluent API

c#, ef-code-first, ef-fluent-api, entity-framework, one-to-one

Solution

EF Code First supports `1:1` and `1:0..1` relationships. The latter is what you are looking for ("one to zero-or-one").

Your attempts at fluent are saying required on both ends in one case and optional on both ends in the other.

What you need is optional on one end and required on the other.

Here's an example from the Programming E.F. Code First book

modelBuilder.Entity<PersonPhoto>()
.HasRequired(p => p.PhotoOf)
.WithOptional(p => p.Photo);

The `PersonPhoto` entity has a navigation property called `PhotoOf` that points to a `Person` type. The `Person` type has a navigation property called `Photo` that points to the `PersonPhoto` type.

In the two related classes, you use each type's primary key, not foreign keys. i.e., you won't use the `LoyaltyUserDetailId` or `PIIUserId` properties. Instead, the relationship depends on the `Id` fields of both types.

If you are using the fluent API as above, you do not need to specify `LoyaltyUser.Id` as a foreign key, EF will figure it out.

So without having your code to test myself (I hate doing this from my head)... I would translate this into your code as

public class PIIUser
{
    public int Id { get; set; }    
    public LoyaltyUserDetail LoyaltyUserDetail { get; set; }
}

public class LoyaltyUserDetail
{
    public int Id { get; set; }
    public double? AvailablePoints { get; set; }    
    public PIIUser PIIUser { get; set; }
}

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
  modelBuilder.Entity<LoyaltyUserDetail>()
  .HasRequired(lu => lu.PIIUser )
  .WithOptional(pi => pi.LoyaltyUserDetail );
}

That's saying LoyaltyUserDetails `PIIUser` property is required and PIIUser's `LoyaltyUserDetail` property is optional.

You could start from the other end:

modelBuilder.Entity<PIIUser>()
.HasOptional(pi => pi.LoyaltyUserDetail)
.WithRequired(lu => lu.PIIUser);

which now says PIIUser's `LoyaltyUserDetail` property is optional and LoyaltyUser's `PIIUser` property is required.

You always have to use the pattern HAS/WITH.

HTH and FWIW, one to one (or one to zero/one) relationships are one of the most confusing relationships to configure in code first so you are not alone! :)

Problem

We want to use one to one optional relationship using Entity Framework Code First. We have two entities. ``` public class PIIUser { public int Id { get; set; } public int? LoyaltyUserDetailId { get; set; } public LoyaltyUserDetail LoyaltyUserDetail { get; set; } } public class LoyaltyUserDetail { public int Id { get; set; } public double? AvailablePoints { get; set; } public int PIIUserId { get; set; } public PIIUser PIIUser { get; set; } } ``` `PIIUser` may have a `LoyaltyUserDetail` but `LoyaltyUserDetail` must have a `PIIUser`. We tried these fluent approach techniques. ``` modelBuilder.Entity<PIIUser>() .HasOptional(t => t.LoyaltyUserDetail) .WithOptionalPrincipal(t => t.PIIUser) .WillCascadeOnDelete(true); ``` This approach didn't create `LoyaltyUserDetailId` foreign key in `PIIUsers` table. After that we tried the following code. ``` modelBuilder.Entity<LoyaltyUserDetail>() .HasRequired(t => t.PIIUser) .WithRequiredDependent(t => t.LoyaltyUserDetail); ``` But this time EF didn't create any foreign keys in these 2 tables. Do you have any ideas for this issue? How can we create one to one optional relationship using entity framework fluent api?

Original source