Question

I’m trying to use the before_validation callback to adjust the number of child objects for a record, but for some reason, its not working the way I expect.

LineItem class:

 before_validation :adjust_enrollment_count

 def adjust_enrollment_count
    if enrollments.size < quantity 
      (enrollments.size+1..quantity).each do |li|
        self.enrollments.build(variant: self.variant)
      end
      #self.save
    elsif enrollments.size > quantity
      enrollments.delete_if do |e| 
        enrollments.size > quantity
      end
    end
  end

What happens is that it creates the correct number of Enrollment objects as children to the LineItem, but the Variant gets set to nil (even though the LineItem has a variant defined).

Things I’ve tried:

• Explicitly saving the line_item or the enrollment
• “pry”ing into the callback and running the code manually (this actually worked the way
  I expected!)
• Verifying that “self” referred to the LineItem and not the closure

Is there something about the callback lifecycle that I’m missing? Is there a better way to adjust the number of Enrollment objects as the quantity changes on the LineItem?

Answer 1

Probably variant is not an accessible field of the Enrollment class. Try this way (also shortened)

def adjust_enrollment_count
  while enrollments.size < quantity 
    self.enrollments.build(variant_id: self.variant) # note: variant_id
  end
  while enrollments.size > quantity
    enrollments.pop # or .shift to delete from the head of the list
  end
  # don't save in a lifecycle callback, or you'll get in an awful loop
end

EDIT: a different take

def add_enrollment
  enrollments.build(variant_id: variant)
end

def adjust_enrollment_count
  enrollments.slice!(quantity, enrollments.size)
  add_enrollment while enrollments.length < quantity
end