Question

I have a problem when defining a model in EF5. I want make the ParentId as a Foreign Key in the database but I’m getting an error:

System.Data.Entity.Edm.EdmAssociationType: : Multiplicity conflicts with the referential constraint in Role ‘Category_Parent_Target’ in relationship ‘Category_Parent’. Because all of the properties in the Dependent Role are non-nullable, multiplicity of the Principal Role must be ‘1’.

Here is my Model:

public class Category
{
    public Category()
    {
        this.Childs = new HashSet<Category>();
    }
    public int CategoryId { get; set; }
    public string Name { get; set; }
    public int ParentId { get; set; }
    public virtual Category Parent { get; set; }
    public virtual ICollection<Category> Childs { get; set; } 
}

and

modelBuilder.Entity<Category>()
            .HasOptional(c => c.Parent)
            .WithMany(c => c.Childs)
            .HasForeignKey(d => d.ParentId);

Answer 1

If a Category may or may not have a Parent, then the ParentId may or may not have a value.

So, make ParentId nullable:

public int? ParentId { get; set; }