c# - Unable to determine the principle end of the relationship - multiple added entities may have the same primary key

Question

Sets have Cards and Sets. Here's what I have in my model, using EF Code First:

public class Set
{
    // Primitive Properties 
    [Required]
    [Key]
    public virtual int SetId { get; set; }

    // Navigation Properties 
    [Required]
    public virtual List<Set> Sets { get; set; }

    // Navigation Properties
    [ForeignKey("ParentSet")]
    public int ParentSetId { get; set; }
    public virtual Set ParentSet { get; set; }
}

Then for Cards:

public class Card
{
    // Primitive Properties
    [Required]
    [Key]
    public virtual int CardId { get; set; }

    // Navigation Properties
    [Required]
    [ForeignKey("ParentSet")]
    public int ParentSetId { get; set; }
    public virtual Set ParentSet { get; set; }
}

I'm trying to rebuild the database using 'update-database' from the package manager console and this is the error I'm getting:

Unable to determine the principal end of the 'App.Core.Set_ParentSet' relationship. Multiple added entities may have the same primary key.

Any idea why?

Answer 1

To me having this in the Set entity doesn't make sense. It can't refer to itself and be required

// Navigation Properties 
[Required]
public virtual List<Set> Sets { get; set; }

If a Set HAS to have a list of Sets, how can you create the first Set?

Note that from what's written in the error, it has nothing to do with the Card class