validates_uniqueness_of field scoped to a has_one relationship

I have the following models:

class Section < ActiveRecord::Base
  belongs_to :course                                                          
  has_one :term, :through => :course
end

class Course < ActiveRecord::Base
  belongs_to :term
  has_many :sections
end

class Term < ActiveRecord::Base
  has_many :courses
  has_many :sections, :through => :courses
end

I would like to be able to do the following in my Section model (call_number is a field in Section):

validates_uniqueness_of :call_number, :scope => :term_id

This obviously doesn't work because Section doesn't have term_id, so how can I limit the scope to a relationship's model?

I tried creating a custom validator for Section to no avail (doesn't work when I create a new Section with the error "undefined method 'sections' for nil:NilClass"):

def validate_call_number
  if self.term.sections.all(:conditions => ["call_number = ? AND section开发者_开发技巧s.id <> ?", self.call_number, self.id]).count > 0
    self.errors[:base] << "Call number exists for term"
    false
  end
  true
end

Thanks a lot!

---

Assuming your validation code is correct, why don't you simply add a check for term existence?

def validate_call_number
  return true if self.term.nil? # add this line
  if self.term.sections.all(:conditions => ["call_number = ? AND sections.id <> ?", self.call_number, self.id]).count > 0
    self.errors[:base] << "Call number exists for term"
    false
  end
  true
end