Self referencing / parent-child relationship in Entity Framework
ef-code-first, entity-framework, entity-framework-4.1, parent-child, self-reference
Solution
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.
Problem
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?