def touch_atomic_updates(field = nil)
  updates = atomic_updates
  return {} unless atomic_updates.key?("$set")
  touches = {}
  updates["$set"].each_pair do |key, value|
    key_regex = /updated_at|u_at#{"|" + field if field.present?}/
    touches.merge!({ key => value }) if key =~ key_regex
  end
  { "$set" => touches }
end

# the embedded document generated this atomic update
# atomic_updates = {"$set"=>{"email_suppressions.0.notified_at"=>2010-10-10 10:00:00 UTC}}
# because the parent is touched, the field is `nil`
# the regex will be /updated_at|u_at/
# this does not match the key =~ key_regex
# the resulting $set will thus become: `{"$set"=>{}}`
# effectively removing the atomic update for an embedded document
# when the field is custom