class Submission < ActiveRecord::Base #Set up how the model works belongs_to :contest belongs_to :subject #Validate the field we're talking about in this post: validates_presence_of :subject_id, :on => :create, :if => :needs_subject?
.. And later I defined needs_subject? as follows:
def needs_subject? if self.contest.subjects.any? return true else return false end end end #End the model.
So this allowed me to only check if there's a subject if there SHOULD BE a subject.
Hope this helps someone.