Drills into related Mongoid models to create a single error message.

err_drill.rb

##
# Drills into related Mongoid models to create a single error message.
#
# @param obj [Object] An instance of a Mongoid model class.
# @param prefix [String] The class/attribute name prefix (for recursive calls).
#
# @return [String] A string of concatenated error messages.
#
def err_drill(obj, prefix = nil)
  prefix = "#{obj.class.to_s.downcase}." if prefix.nil?

  # Error messages for this object.
  msg = obj.errors.messages.map do |error|
    "(#{prefix}#{error[0].to_s.singularize}) #{error[1].join(", ")}"
  end.join("; ")

  # Look over the relations and concatenate their error messages.
  obj.relations.each do |relation|
    metadata = relation[1]

    # Only drill into "child" relations.
    if metadata.macro.match /(_many|_one)/
      children = obj.send(metadata.name.to_sym)
      children = [children] unless children.is_a? Array

      children.each do |child|
        if child.respond_to?(:errors) && child.errors.messages.empty? == false
          msg << "; " << err_drill(child, "#{prefix}#{metadata.name.to_s.singularize}.")
        end
      end
    end
  end

  return msg
end