lib/parse/model/associations/relation_collection_proxy.rb
def add_unique!(*items)
      return false unless @delegate.respond_to?(:op_add_relation!)
      items = items.flatten.parse_pointers
      @delegate.send :op_add_relation!, @key, items
    end
def add_unique!(*items)
      return false unless @delegate.respond_to?(:op_add_relation!)
      items = items.flatten.parse_pointers
      @delegate.send :op_add_relation!, @key, items
    end
Atomically add a set of Parse::Objects to this relation.
 This is done by making the API request directly with Parse server; the
 local object is not updated with changes.