Self referencing / parent-child relationship in Entity Framework - entity-framework

Question

I read quite a number of posts of programmers that run into the Unable to determine a valid ordering for dependent operations. Dependencies may exist due to foreign key constraints, model requirements, or store-generated values -exception when using a self-referencing relationship in Entity Framework.
I am trying to get a parent-child relationship to work:
public class Category {
public int CategoryId { get; set; }
public string Name { get; set; }
public int ParentId { get; set; }
public Category Parent { get; set; }
public List<Category> Children { get; set; }
}
This is the configuration I use (Fluent API):
Property(c => c.ParentId).IsOptional();
HasMany(c => c.Children).WithOptional(c => c.Parent).HasForeignKey(c => c.ParentId);
//HasOptional(c => c.Parent).WithMany(c => c.Children).HasForeignKey(c => c.ParentId);
Both the HasMany() and HasOptional() configurations result in a "Unable to determine a valid ordering for dependent operations..." exception when I try to save a new category like this:
context.Categories.Add(new Category { Name = "test" });
I don't understand why EF doesn't insert the Category with a null parentId. The database allows the ParentId foreign key to be null.
Would you be able to tell me how to do this?

Answer 1

You must define the ParentId in the category class as nullable to use it as the foreign key property for an optional relationship:
public int? ParentId { get; set; }
An int property cannot take the value null and therefore cannot represent a NULL as value in a database column.

Answer 2

Since someone asked in a comment about doing this with attributes. You can also utilize data annotations to set this up. Using the same example as above:
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
public class Category {
// You can also add [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
// as an attribute, if this field is to be generated by the database
[Key] // Define this as the primary key for the table
public int CategoryId { get; set; }
public string Name { get; set; }
[ForeignKey(nameof(Parent))] // Link the Parent object to the ParentId Foreign Key
public int? ParentId { get; set; }
public Category Parent { get; set; }
public List<Category> Children { get; set; }
}
This is tested and works in EF 6.