Entity Framework 0..1 to 0 relation

Question

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

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

    public int? First_Id { get; set; }

    [ForeignKey("First_Id")]
    public First First { get; set; }
}

public class SecondMapping : EntityTypeConfiguration<Second>
{
    public SecondMapping () 
        : base()
    {
        this.HasOptional(s => s.First)
            .With ... ???
    }
}

Second may have a reference to First. But First never has a reference to Second. Is it possible to apply this mapping with Entity Framework 4.1?

EDIT: Previously, that was my solution:

this.HasOptional(s => s.First)
    .WithOptionalDependent()
    .WillCascadeOnDelete(false);

Second could contain one instance of First (dependent on some kind of Usage-Attribute). First doesn't contain any instance of Second.

Answer 1

One-to-one relation is possible only if foreign key is also primary key of dependent entity. So the correct mapping is:

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

class Second
{
    [Key, ForeignKey("First")]
    public int Id { get; set; }
    public First First { get; set; }
}

The reason is that to enforce one-to-one relation in the database foreign key must be unique in the Second entity. But entity framework doesn't support unique keys - the only unique value for EF is primary key. This is limitation of EF.

There is workaround described on Morteza Manavi's blog. Workaround is based on mapping association as one-to-many and enforcing uniqueness in database by introducing unique constraints in custom database initializer.

Answer 2

If you're trying to achieve a 1-to-1 relationship, where there is at the most only one Second entity associated to a First entity, and where there is no reverse property try the following:

class First
{
    [Key]
    public Guid FirstId { get; set; }
}

class Second
{
    [Key]
    public Guid FirstId { get; set; }
    public First First { get; set; }
}

class SecondMapping : EntityTypeConfiguration<Second>
{
    public SecondMapping()
    {
        this.HasRequired(s => s.First);
    }
}

You can however use a separate First_id column to do this kind of association, but then you would be effectively creating a 1-to-N relationship. It can be 'forced' to be 1-to-1 via a UNIQUE constraint, but you won't be able to create a reverse property due to a limitation in EF (as Ladislav mentioned):

class SecondMapping : EntityTypeConfiguration<Second>
{
    public SecondMapping()
    {
        this.HasRequired(s => s.First).WithMany().HasForeignKey("First_id");
    }
}