Code First Optional One-To-One Relationship

Question

Writing a model for situation where I have two tables which are customers and users. Each user record might have an optional related customer record and vice versa, but none of them is a must. I figured out that FK Associations are not what I need, but Independent Associations are. But I just can find a way to make it work, I keep getting the 'Unable to determine the principal end...The principal end of this association must be explicitly configured using either the relationship fluent API or data annotations.' exception.

My models are very simple:

public class User
{
    [Key]
    public int          Id              { get; set; }
    [StringLength(20)]
    public string       CustomerId      { get; set; }
    public string       Password        { get; set; }
    public bool         Locked          { get; set; }

    //[ForeignKey("CustomerId")]
    public virtual Customer Customer    { get; set; }
}

public class Customer
{
    [Key]
    [Column("Id", TypeName = "nvarchar")]
    [StringLength(20)]
    public string       Id              { get; set; }   //  nvarchar    20
    [Required]
    public string       GivenName       { get; set; }   //  nvarchar    100
    [Required]
    public string       Surname         { get; set; }   //  nvarchar    100

    //[InverseProperty("Customer")]
    public virtual User User            { get; set; }
}

I've tried to add the ForeignKeyAttribute and InversePropertyAttribute, which are currently commented out, but they didn't help either. I would prefer to use data annotations and not fluent API, if it's possible in my case.

Answer 1

In one-to-one relation one end must be principal and second end must be dependent. Principal end is the one which will be inserted first and which can exist without the dependent one. Dependent end is the one which must be inserted after the principal because it has foreign key to the principal. When configuring one-to-one relationships, Entity Framework requires that the primary key of the dependent also be the foreign key.This problem is most easily solved by using a ForeignKey annotation on the dependent class to identify that it contains the foreign key. In your case, Customer could be the dependent and its key, Customer.UserId, should also be the foreign key. But both Keys must be declared using the same type:

public class User
{
   [Key]
   public int  Id  { get; set; }

   public virtual Customer Customer { get; set; }
}

public class Customer
{
   [Key, ForeignKey("User")]
   public int  UserId { get; set; }

   public virtual User User{ get; set; } 
}

I don't know how to resolve your problem using Data Annotations, but if you want to use Fluent Api, I think the configuration of the relationship would be like this:

 modelBuilder.Entity<User>().HasOptional(u => u.Customer).WithOptionalPrincipal(c => c.User);

Update

I understand your escenario, but if you have the same columns that you show in your model, I think you should have a one-to-many relationship mapped in DB instead one-to-one. Try to map your relationship this way:

public class User
{
    [Key]
    public int Id { get; set; }

    public string Password { get; set; }
    public bool Locked { get; set; }

    public string CustomerId { get; set; }

    [ForeignKey("CustomerId")]
    public virtual Customer Customer { get; set; }
}

public class Customer
{
    [Key]
    [Column("Id", TypeName = "nvarchar")]
    [StringLength(20)]
    public string Id { get; set; }   //  nvarchar    20
    [Required]
    public string GivenName { get; set; }   //  nvarchar    100
    [Required]
    public string Surname { get; set; }   //  nvarchar    100

    public virtual  ICollection<User> Users { get; set; }
}

Remember map your properties with the same column'names that you have in DB.