Where put the referential integrity in the MVC structure of CI.

[eluser]BrianDHall[/eluser]
Your validation would be in your model, presumably in your add() method - you validate that add can be called with the parameters submitted in that given context, and if not either return false, or send back an error string, or blatantly throw an error.

I do something similar with an edit() function in a model - there I make sure the presently logged in user should actually be editing that item before I save the results.