Entity Framework Code First One-to-One Required-Required Relationship - entity-framework

Question

When using Entity Framework Code First 4.3.1 it is possible to create relationships with a multiplicity of 1-to-1. That is, one entity on each end of the relationship.
It is possible to configure 1-to-1 relationships to be required-required or required-optional ^. However, when I switch between the two I do not see any differences in:
The database schema generated. I am targeting SQL Server 2008.
The runtime behaviour of EF.
As such, I am able to create a RequiredPrincipalAs record without a corresponding RequiredDependentAs record, despite the relationship being configured as required-required. This seems to contradict the documentation for HasRequired(...):
Configures a required relationship from this entity type. Instances of the entity type will not be able to be saved to the database unless this relationship is specified. The foreign key in the database will be non-nullable.
http://msdn.microsoft.com/en-us/library/gg671317
The required-required relationship entities:
public class RequiredPrincipalA
{
public int Id { get; set; }
public virtual RequiredDependentA DependentA { get; set; }
}
public class RequiredDependentA
{
public int Id { get; set; }
public virtual RequiredPrincipalA PrincipalA { get; set; }
}
The required-optional relationship entities:
public class RequiredPrincipalB
{
public int Id { get; set; }
public virtual OptionalDependentB DependentB { get; set; }
}
public class OptionalDependentB
{
public int Id { get; set; }
public virtual RequiredPrincipalB PrincipalB { get; set; }
}
The DbContext and model configuration:
public class AppContext : DbContext
{
public DbSet<RequiredPrincipalA> PrincipalAs { get; set; }
public DbSet<RequiredDependentA> DependentAs { get; set; }
public DbSet<RequiredPrincipalB> PrincipalBs { get; set; }
public DbSet<OptionalDependentB> DependentBs { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<RequiredPrincipalA>()
.HasRequired(o => o.DependentA)
.WithRequiredPrincipal(o => o.PrincipalA);
modelBuilder.Entity<RequiredPrincipalB>()
.HasOptional(o => o.DependentB)
.WithRequired(o => o.PrincipalB);
}
}
The test code:
Database.SetInitializer(new DropCreateDatabaseAlways<AppContext>());
using (var ctx = new AppContext())
{
ctx.Database.Initialize(force: false);
ctx.PrincipalAs.Add(new RequiredPrincipalA());
ctx.PrincipalBs.Add(new RequiredPrincipalB());
ctx.SaveChanges();
}
I am aware I could add a [Required] data attribute to the navigation properties of RequiredPrincipalA.DependentA and RequiredDependentA.PrincipalA. This would cause EF validation to prevent the scenario above. However, I do not want to do this because it also validates the navigation property is populated when updating an existing entity. This means the application has to pre-fetch the entity at the other end of the relationship for every update.
Why do I not see any difference in the behaviour of EF just when changing a relationship between required-required and required-optional?
^ Note that optional-optional is also supported but this doesn't form part of my question. There are obvious differences in the generated database schema and runtime behaviour when an optional-optional relationship is configured.

Answer 1

I don't know why required-required is allowed for this case but it cannot exist in the database because relation is build on primary keys. Required-required means that A cannot be inserted if related B doesn't exist and B cannot be inserted if related A doesn't exist => neither A or B can be inserted.
Database relation has always principal and dependent entity - principal can always exist without dependent.
Real required-required in EF can be achieved only when both A and B are mapped to the same table (table splitting) because in such case they are both inserted with single insert command.

Answer 2

Not really an answer but I have more to say than will fit in comments. But you know, I write 900 page books...it's just how I roll. :)
Oddly I would expect the fluent configuration to behave the same way as the data annotation and am confused that it's not doing it. (I've pinged Rowan Miller with a link to this thread to get his feedback.) And the behavior I mean is: validating the constraint during SaveChanges.
On the database side, I'm with Ladislav.In the model, EF defines the 1:1 using the keys of the related entities. But in the database, you can't have FKs in both tables, so only the dependent table in the database will require that constraint that it's PK maps to an existing PK in the principal table.
And finally, I understand your reason for not wanting EF to enforce the relationship if you aren't going to always deal with teh full graph. I think 1:1 relationships are the most confusing of the EF relationship mappings and I always find myself having to go back for reminders of the rules and how things should work.

Answer 3

Old question. But since EF6 is still used and even available for .Net standard and this issue can be a real nuisance, I think it's worth mentioning something I couldn't find in other answers.
It is true that both HasRequired - WithRequiredPrincipal and HasOptional - WithRequired produce the same database schema and the same runtime behavior. That is, with both mappings it's possible to save a principal without a dependent entity and to remove the dependent later. So much for HasRequired.
But there is a way to make EF validate the required relationship when creating the entities, which is by simply adding a [Required] attribute:
public class RequiredPrincipalA
{
public int Id { get; set; }
[Required] // <== here
public virtual RequiredDependentA DependentA { get; set; }
}
public class RequiredDependentA
{
public int Id { get; set; }
public virtual RequiredPrincipalA PrincipalA { get; set; }
}
As said, only when creating the entities. It's still possible to set RequiredPrincipalA.RequiredDependentA = null and save it successfully. But I think that, fortunately, the likelihood of that happening in code is far lower than forgetting to set the required dependent.