jsonapi rb关系示例

我试图得到一个JSON,它看起来像这样

{
   "data" : {
     "type": "field_definition",
     "id": 5,
     "attributes": {
        "specifier": "foo",
        "entity_type": "bar"
     }
     "relationships": {
        "active_collections" : [1,2,3]
     }
   }
}

  class Test
     attr_reader :id,
              :specifier,
              :entity_type,
              :active_collections

      def initialize
        @id = 5
        @specifier = "foo"
        @entity_type = "bar"
        @active_collections = [1,2,3]
      end
    end

class SerializableFieldCollection < JSONAPI::Serializable::Resource
  type 'field_collection'

  attributes :specifier, :entity_type

  has_many :active_collections 
end

  def index
    render jsonapi: Test.new,
           class:   SerializableFieldCollection,
           status:  200   
  end


{"data"=>
  {"id"=>"5", "type"=>"field_collection", "attributes"=>{"specifier"=>"foo", "entity_type"=>"bar"}, "relationships"=>{"active_collections"=>{"meta"=>{"included"=>false}}}}}

有人能告诉我如何使用jsonapi rb gem中的relationships/has_many函数吗?