Will one-to-one foreign key associations be supported in EF v-Next? - entity-framework

Question

I have entities A and B and I want to create 2 distinct 1-1 associations between A and B. A should play the role as principal. Like this:
public class A
{
public int Id {get; set;}
public B B1 {get; set;}
public B B2 {get; set;}
}
public class B
{
public int Id {get; set;}
}
Since EF does not support one-to-one foreign key associations I cannot create a working model/database with EF. To my this sounds like a serious limitation. Are there any plans to support such associations in an upcoming version of EF?
What is the best workaround for to get this working. I know about creating two one-2-many associations. However, that would make B the principal and gives me problems with cascading deletes.
Thanks for replying to my question. Below is an example of what I want to do, i.e., create two (or more) 1-to-1 associations between an entity A and another entity B. Is this something that EF could support in vNext, or else, why would it be a bad idea?
Thanks again,
Merijn
public class A
{
public int Id {get; set;}
public int B1_Id {get; set;}
public B B1 {get; set;}
public int B2_Id {get; set;}
public B B2 {get; set;}
}
public class B
{
public int Id {get; set;}
}
public class SampleContext : DbContext
{
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<A>().HasKey(c => c.Id);
modelBuilder.Entity<B>().HasKey(c => c.Id);
modelBuilder.Entity<A>().HasRequired(c => c.B1).WihOptional().ForeignKey(x=>x.B1_Id);
modelBuilder.Entity<A>().HasRequired(c => c.B2).WihOptional().ForeignKey(x=>x.B2_Id);
}
}

Answer 1

If "v-Next" is Entity Framework 6, then no, it apparently won't support one-to-one foreign key associations, as you can see on the roadmap for all features planned for EF 6.
You can also see that Unique Constraint support is not on the roadmap and still marked as "Under Review" on UserVoice.
Because a one-to-one foreign key association is basically a one-to-many association with a unique constraint on the foreign key column I would expect that one-to-one FK associations won't be implemented before Unique Constraint support is available. It's especially required if you want that A is the principal in your two relationships. Currently EF does not support relationships where the principal's key is not the primary key but some column with unique constraint.
In this blog post the feature is described and mentioned that it is "postponed", so let's hope for EF 7.

Answer 2

Perhaps it is a terminology issue.
In Code first EF, EF doesnt allow you to have 1:1 relationships with Principal and Dependent both with foreign keys to each other
or with the dependent having its own primary key unrelated to Principal.
With your example it looks like that it is a case of 2 navigation properties required.
And strictly speaking it is not 1:1. since you have 2 relationships to the same table.
you have 2 relationships of type 1:1.. EF sees this as many to 1.
If you have a true 1:1 relationship, EF will want the dependent to have the same Primary Key as the primary.
You can define Multiple NAVIGATION properties on Both Principle and dependent, which result in indexes.
So you may wish to investigate Many to 1 configurations
If you want the Primary to have an OPTINAL Foreign Key at DB level, You would need to ADD this FK later during migration or with script.
But arguably this is best seen as business logic/rule check rather than an OPTIONAL FK on principal.
So yes there are limitations in matching exactly what is possible on the DB.
But it is questionable is actually necessary in a code first scenario.
Neat trick here btw is to model in DB exactly what you want on Code first.
There use the EF Powertool nuget to reerse engineer Codefirst from DB.
EG mini DB with just the desired table relationships.
make a new project in Solution. Install Entity Framework Powertools.
Then use right click option in new project to "reverse engineer code first from DB".
It shows how to build that in code first if it can.... :-)
What I think you wanted to achieve... see code sample (sorry if I misunderstood the point your are making) code should execute if NUGET is loaded
using System.Data.Entity;
namespace EF_DEMO
{
class FK121
{
public static void ENTRYfk121(string[] args)
{
var ctx = new Context121();
ctx.Database.Create();
System.Console.ReadKey();
}
}
public class Main
{
public int MainId { get; set; }
public string BlaMain { set; get; }
public int? Sub1Id { set; get; } // Must be nullable since we want to use EF foreign key
public int? Sub2Id { set; get; } // Must be nullable since we want to use EF foreign key
public virtual Sub Sub1 { get; set; } // Reverse navigation
public virtual Sub Sub2 { get; set; } // Reverse navigation
// you may also need
public virtual ICollection<Sub> Subs { get; set; }
}
public class Sub
{
public int SubId { get; set; } // Deliberately DIFFERENT KEY TO MAIN.... not 1:1 so this is possible
public string blasub { set; get; }
public int MainId { set; get; } //set in API , this the FK
public virtual Main Main { get; set; } // van to Principal
}
public class Context121 : DbContext
{
static Context121()
{
Database.SetInitializer(new DropCreateDatabaseIfModelChanges<Context121>());
}
public Context121()
: base("Name=Demo") { } // webconfig required to match
public DbSet<Main> Mains { get; set; }
public DbSet<Sub> Subs { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Main>().HasKey(t => t.MainId)
.HasOptional(t => t.Sub1)
.WithMany()
.HasForeignKey(t=>t.Sub1Id) ; // tell EF the field is in POCO, use this please, otherwise it will create it.
modelBuilder.Entity<Main>()
.HasOptional(t => t.Sub2).WithMany()
.HasForeignKey(t=>t.Sub2Id);
modelBuilder.Entity<Sub>()
.HasKey(t => t.SubId)
.HasRequired(q => q.Main)
.WithMany()
.HasForeignKey(t => t.MainId);
}
}
}
WEBCONFIG....
<connectionStrings>
<add name="Demo" connectionString="Data Source=localhost;Initial Catalog=Demo;Integrated Security=True;MultipleActiveResultSets=True;App=EntityFramework"
providerName="System.Data.SqlClient" />
</connectionStrings>

Answer 3

Explain what problem do you need to resolve? This is sample of one-to-one mapping in EF 5.0
class Program
{
static void Main(string[] args)
{
using (var context = new SampleContext())
{
var mainEntity = new MainEntity();
mainEntity.DetailEntity = new DetailEntity();
context.SaveChanges();
}
}
}
public class SampleContext : DbContext
{
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<MainEntity>().HasKey(c => c.Id);
modelBuilder.Entity<DetailEntity>().HasKey(c => c.Id);
modelBuilder.Entity<MainEntity>().HasOptional(c => c.DetailEntity).WithRequired(p => p.MainEntity);
modelBuilder.Entity<DetailEntity>().HasRequired(c => c.MainEntity).WithOptional(p => p.DetailEntity);
}
public virtual DbSet<MainEntity> MainEntities { get; set; }
public virtual DbSet<DetailEntity> DetailEntities { get; set; }
}
public class MainEntity
{
public int Id { get; set; }
public DetailEntity DetailEntity { get; set; }
}
public class DetailEntity
{
public int Id { get; set; }
public MainEntity MainEntity { get; set; }
}