JSON error thoughts

card.rb

class Card < ActiveRecord::Base
  attr_accessible :reference, :scripture, :subject

  validates :scripture, presence: true, length: { minimum: 20 }
  validate :at_least_one_subject_or_reference

  def self.updated_since(time)
    if time
      Card.where("updated_at >= ?", time)
    else
      Card.all
    end
  end

  private

  def at_least_one_subject_or_reference
    if String(subject).empty? && String(reference).empty?
      errors[:handle] << "Please include a reference or subject"
    end
  end
end

irb_sample.rb

>> c = Card.new
=> #<Card id: nil, subject: nil, scripture: nil, reference: nil, created_at: nil, updated_at: nil>
>> c.save
   (0.4ms)  BEGIN
   (0.5ms)  ROLLBACK
=> false
>> c.errors.as_json
=> {:scripture=>["can't be blank", "is too short (minimum is 20 characters)"], :handle=>["Please include a reference or subject"]}

response.json

{"errors": { "scripture": ["can't be blank","is too short (minimum is 20 characters)"],"handle":["Please include a reference or subject"]}}