Symmetric self-referential many-to-many

Question

I'm trying to model a hierarchy of categories where a category can have multiple parents ( an overlapping tree model as described in this book )

I have the following tables

video_categories
  int     id
  string  name

video_category_links
  int     parent_id  (foreign_key to video_categories.id)
  int     child_id   (foreign_key to video_categories.id)

and an index on [ parent_id, child_id ] for uniqueness

The issue with this setup is that A can be parent of B, and B can be parent of A at the same time

Is there a way to specify a db constraint to avoid this or should I ensure it at the application level?

Thank you very much