EF Code First - 1-to-1 Optional Relationship

Question

I want to map an optional 1-to-1 relationship in an existing database with EF Code First.

Simple schema:

User  Username  ContactID  Contact  ID  Name 

Obviously ContactID joins to Contact.ID. The ContactID field is nullable so the relationship is optional - 0 or 1, never many.

So how do I specify this relationship in EF Code First, with this existing schema?

Here's what I've tried so far:

public class User {     [Key]     public string Username { get; set; }     public int? ContactID { get; set; }      [ForeignKey("ContactID")]     public virtual Contact Contact { get; set; } }  public class Contact {     [Key]     public int ID { get; set; }     public string Name { get; set; }      public virtual User User { get; set; } }  modelBuilder.Entity<User>().HasOptional<Contact>(u=> u.Contact)     .WithOptionalDependent(c => c.User); 

I get the following Exception:

  System.Data.Edm.EdmAssociationEnd: : Multiplicity is not valid in Role  'User_Contact_Source' in relationship 'User_Contact'. Because the Dependent  Role properties are not the key properties, the upper bound of the multiplicity  of the Dependent Role must be *. 

Answer 1

One solution would be;

public class User {     [Key]     public string Username { get; set; }      public virtual Contact Contact { get; set; } }  public class Contact {     [Key]     public int ID { get; set; }     public string Name { get; set; }      public virtual User User { get; set; } }          modelBuilder.Entity<User>()             .HasOptional<Contact>(u => u.Contact)             .WithOptionalDependent(c => c.User).Map(p => p.MapKey("ContactID")); 

You set only your navigational objects in your POCOs and instead you use fluent API to map your key to the correct column.