One to one optional relationship using Entity Framework Fluent API

Question

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?

Solution 1

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! :)

Solution 2

Just do like if you have one-to-many relationship between LoyaltyUserDetail and PIIUser so you mapping should be

modelBuilder.Entity<LoyaltyUserDetail>()
       .HasRequired(m => m.PIIUser )
       .WithMany()
       .HasForeignKey(c => c.LoyaltyUserDetailId);

EF should create all foreign key you need and just don't care about WithMany !

Solution 3

public class User
{
    public int Id { get; set; }
    public int? LoyaltyUserId { get; set; }
    public virtual LoyaltyUser LoyaltyUser { get; set; }
}

public class LoyaltyUser
{
    public int Id { get; set; }
    public virtual User MainUser { get; set; }
}

        modelBuilder.Entity<User>()
            .HasOptional(x => x.LoyaltyUser)
            .WithOptionalDependent(c => c.MainUser)
            .WillCascadeOnDelete(false);

this will solve the problem on REFERENCE and FOREIGN KEYS

when UPDATING or DELETING a record

Solution 4

There are several things wrong with your code.

A 1:1 relationship is either: PK<-PK, where one PK side is also an FK, or PK<-FK+UC, where the FK side is a non-PK and has a UC. Your code shows you have FK<-FK, as you define both sides to have an FK but that's wrong. I recon PIIUser is the PK side and LoyaltyUserDetail is the FK side. This means PIIUser doesn't have an FK field, but LoyaltyUserDetail does.

If the 1:1 relationship is optional, the FK side has to have at least 1 nullable field.

p.s.w.g. above did answer your question but made a mistake that s/he also defined an FK in PIIUser, which is of course wrong as I described above. So define the nullable FK field in LoyaltyUserDetail, define the attribute in LoyaltyUserDetail to mark it the FK field, but don't specify an FK field in PIIUser.

You get the exception you describe above below p.s.w.g.'s post, because no side is the PK side (principle end).

EF isn't very good at 1:1's as it's not able to handle unique constraints. I'm no expert on Code first, so I don't know whether it is able to create a UC or not.

(edit) btw: A 1:1 B (FK) means there's just 1 FK constraint created, on B's target pointing to A's PK, not 2.

Solution 5

Try adding the ForeignKey attribute to the LoyaltyUserDetail property:

public class PIIUser
{
    ...
    public int? LoyaltyUserDetailId { get; set; }
    [ForeignKey("LoyaltyUserDetailId")]
    public LoyaltyUserDetail LoyaltyUserDetail { get; set; }
    ...
}

And the PIIUser property:

public class LoyaltyUserDetail
{
    ...
    public int PIIUserId { get; set; }
    [ForeignKey("PIIUserId")]
    public PIIUser PIIUser { get; set; }
    ...
}

View more solutions

Comments

[Ilkay Ilknur]

We tried data annotations before all of the these fluent API approaches. But data annotations didn't work. if you add data annotations you mentioned above, EF throws this exception => Unable to determine the principal end of an association between the types 'LoyaltyUserDetail' and 'PIIUser'. The principal end of this association must be explicitly configured using either the relationship fluent API or data annotations.

[p.s.w.g]

@İlkayİlknur What happens if you add the ForeignKey attribute to only one end of of the relationship? i.e. only on PIIUser or LoyaltyUserDetail.

[Ilkay Ilknur]

Ef throws the same exception.

[Jennifer Miles]

Doesn't that limit the user on which FK field to pick? (I think he wants that, but he hasn't reported back so I don't know), as it seems he wants to have an FK field present in the class.

[Julie Lerman]

Agreed. It's a limitation of how EF works. 1:1 and 1:0..1 depend on primary keys. Otherwise I think you are straying into the "unique FK" which is still not supported in EF. :( (You're more of a db expert...is that correct...this is really about a unique FK?)And not going to be in upcoming Ef6 as per: entityframework.codeplex.com/workitem/299

[Ilkay Ilknur]

Yes @FransBouma, we wanted to use PIIUserId and LoyaltUserId fields as foreign keys. But EF limits us in this situtation as you and Julie mentioned. Thanks for the responses.

[Willy]

@JulieLerman why do you use WithOptional? What is the different with WithRequiredDependent and WithRequiredOptional?

[Julie Lerman]

WithOptional points to a relationship that is optional, e.g. 0..1 (zero or one) because the OP said "PIIUser may have a LoyaltyUserDetail". And your confusion that leads to the 2nd question is beause you've got one of the terms wrong. ;) Those are WithRequiredDependent and WithRequiredPrincipal. Does that make more sense? Pointing to a required end that is either the dependent (aka "child") or principal aka "parent". Even in a one to one relationship where both are equal, EF needs to refer to one as principal & one as dependent. HTH!

[Julio Borges]

In some situations, particularly when the foreign key is formed by more than one field, you must reference via DataAnnotations which fields are part of the foreign key.

[user808128]

@p.s.w.g Why FK at PIIUser? LoyaltyUserDetail has a FK, not PIIUser. So it must be [Key, ForeignKey("PIIUser")] at PIIUserId property of the LoyaltyUserDetail. Try this

[Thomas Boby]

@user808128 You can place the annotation on either the navigation property or the ID, no difference.

[Zero3]

@ThomasBoby Interesting! Do you have a link for the documentation of this?

[Thomas Boby]

@Zero3 F12, it's in the comment. Or here under "Name"

[Mike Devenney]

Julie Lerman's answer was accepted (and should stay accepted, IMHO) because it answers the question and goes into detail about why it's the correct way with backing detail and discussion. Copy and paste answers are certainly available all over SO and I've used a few myself, but as a professional developers you should be more concerned with becoming a better programmer, not just getting the code to build. That said, jr got it right, albeit a year later.

[Thomas.Benz]

Thank you for the answer. It helps me understand the 1 to zero or to 1 mapping.

[Alex Hope O'Connor]

This answer is out of date, see stackoverflow.com/questions/48394969/…

[Mahmoud Darwish]

@ClickOk I know this is an old question and comment, but this is not the correct answer because it used one to many as a workaround, this doesn't make a one to one or one to zero relation

[Sinjai]

What if you wanted to use data annotations?

[Aaron Queenan]

This won't work as expected. It results in migrations code that doesn't use LoyaltyUserId as a foreign key, so User.Id and LoyaltyUser.Id will end up with the same value, and LoyaltyUserId will be left empty.

[Karthic G]

Can you please share what is the equivalent Fluent API in EF core for this relationship

[Shadam]

This should be the accepted answer :)