Rails Merge child errors with Parent errors

I've got the following two (sanitized/stylized) models:

 class DrivingExam < ActiveRecord::Base
   belongs_to :dmv_rules
   has_many :invigilator_assignments, as: :assignable
   has_many :invigilator, through: :invigilator_assignments

   validate do |record|
     record.invigilator_assignments.each do |i|
       next if i.valid?
       i.errors.full_messages.each do |msg|
         errors.add_to_base(msg)
       end
     end
   end
 end



 class InvigilatorAssignment  < ActiveRecord::Base
   attr_accessible :invigilator_id

   belongs_to :assignable, polymorphic: true
   belongs_to :invigilator

   validates :invigilator_id, presence: true

   validates_each :invigilator do |record, attr, value|
     if record.assignable.is_a?(DrivingExam) && !value.no_scheduling_conflicts?
       record.errors.add attr, "This Invigilator has a scheduling conflict"
     end

   end
 end

These get called from the DrivingExamController by way of:

 if @driving_exam.save

The expected behaviour is that the model should return false on the validation and join the child messages into the parent errors hash and pass this up to the controller.

What happens instead is that the page fails to save (this is good) with a 422(this is weird) and does not pass the messages.

By adding puts statements throughout the above code, I have established that:

1) The if condition within validates_each is successful, and the record.errors array is thus set inside the InvigilatorAssignment model.

2) In the validate do loop the invigilator assignment is valid and has no errors

3) the validate do loop runs before the validates_each loop

So the question is: How do I ensure that the DrivingExam validates InvigilatorAssignment and merges its error messages onto its own error hash.