Parent-Child Relationship in Fluent API

Question

Okay, this is getting ridiculous as this is turning out to be much more difficult than it has any right to be.

If I use my original code with no FluentAPI mapping, I have a ParentID field which is not used, and a new field called Node_ID is used.

public class Node {
  public long ID { get; private set; }
  public long ParentID { get; set; }
  public ICollection<Node> Children { get; set; }
}

Here are my various attempts:

protected override void OnModelCreating(DbModelBuilder mb)
{
  mb.Entity<Node>()
    .HasMany<Node>(h => h.Children)
    .WithOptional()
    .HasForeignKey(h => h.ParentID);
}

DbUpdateException: Unable to determine a valid ordering for dependent operations. Dependencies may exist due to foreign key constraints, model requirements, or store-generated values.

protected override void OnModelCreating(DbModelBuilder mb)
{
  mb.Entity<Node>()
    .HasMany<Node>(h => h.Children)
    .WithOptional()
    .Map(m => m.MapKey("ParentID"));
}

MetadataException: Schema specified is not valid. Errors: (82,6) : error 0019: Each property name in a type must be unique. Property name 'ParentID' was already defined.

[ForeignKey("ParentID")]
public ICollection<Node> Children { get; set; }

protected override void OnModelCreating(DbModelBuilder mb)
{
  mb.Entity<Node>()
    .HasMany<Node>(h => h.Children)
    .WithOptional()
}

DbUpdateException: Unable to determine a valid ordering for dependent operations. Dependencies may exist due to foreign key constraints, model requirements, or store-generated values.

Update

Using the Fluent API code from my first attempt code above (.HasForeignKey), and by making ParentID nullable (public long? ParentID), I have gotten the database to successfully map. Is there any way to do this without making the FK nullable? I would like the key to be 0 when no parent exists. If not, oh well, I will deal.

Making ParentID nullable is correct in my opinion. You cannot set the FK to 0 if no parent exists since there is a foreign key constraint in the DB, so the DB would want to have a row with ID == 0. The only alternative to express that a node has no parent I could think of is a "self reference", so the parent of a node is the node itself - or: ParentID == ID. No clue, if it's possible to map this. I find the nullable way better and easier to understand. — Slauma
The weird thing is that if you use an actual object reference instead of a foreign key (ie. public Node Parent instead of public long ParentID), it will be nonnullable and default to 0. — Alec

Ladislav Mrnka Ladislav Mrnka · Accepted Answer · 2011-06-21T07:29:45

No there is no way to avoid nullable ParentId - you told EF that parent is optional (it must be otherwise you will not be able to use the table) and because of that related FK property must be nullable.

Parent-Child Relationship in Fluent API

Update

1 Answers