How to add a second foreign key to an EF property which is part of a composite foreign key already

Question

I am trying to create an audit trail for a table by creating a duplicate of that table which includes an extra date as part of the primary key.

Here is a simplified example of what I want to do.

My code for the entities is currently as follows:

public class Person
{
    [Key]
    public int ID { get; set; }
    public string Name { get; set; }
    public DateTime DateOfBirth { get; set; }
}

public class GiftIdea
{
    [Key, Column(Order:0), ForeignKey("Person")]
    public int PersonID { get; set; }
    [Key, Column(Order:1)]
    public DateTime RecordDate { get; set; }
    public string Description { get; set; }
    public string Occasion { get; set; }

    //Linked Entities
    public virtual Person Person { get; set; }
}

public class AuditGiftIdea
{
    [Key, Column(Order:0), ForeignKey("GiftIdea")]
    public int PersonID { get; set; }
    [Key, Column(Order:1), ForeignKey("GiftIdea")]
    public DateTime RecordDate { get; set; }
    [Key, Column(Order:2)]
    public DateTime AuditRecordDate { get; set; }
    public string Description { get; set; }
    public string Occasion { get; set; }

    //Linked Entities
    public virtual GiftIdea GiftIdea { get; set; }
}

Ultimately I want to be able to reference a Person from an AuditGiftIdea object without having to go through the attached GiftIdea.

So I tried this:

public class AuditGiftIdeas
{
    [Key, Column(Order:0), ForeignKey("GiftIdea"), ForeignKey("Person")] //Extra Foreign Key
    public int PersonID { get; set; }
    [Key, Column(Order:1), ForeignKey("GiftIdea")]
    public DateTime RecordDate { get; set; }
    [Key, Column(Order:2)]
    public DateTime AuditRecordDate { get; set; }
    public string Description { get; set; }
    public string Occasion { get; set; }

    //Linked Entities
    public virtual GiftIdea GiftIdea { get; set; }
    public virtual Person Person { get; set; } //Access to desired object
}

But I get a compile time error of "Duplicate 'ForeignKey' attribute".

How can I accomplish this setup?

Answer 1

You cannot do that with data annotations because ForeignKeyAttribute is marked with AllowMultiple = false .

But you can configure that with Fluent API:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Entity<AuditGiftIdea>()
        .HasRequired(t => t.Person).WithMany()
        .HasForeignKey(t => t.PersonID)
        .WillCascadeOnDelete(false);

    base.OnModelCreating(modelBuilder);
}

Note that you need to turn cascade delete off, because the new FK is creating a circular cascade path and EF will throw an error if you don't do that. Which in turn means you may have a problems with deleting your Person records - you'll have to manually delete the related records from the AuditGiftIdea table. So think carefully before introducing that FK.