serializer.rb

    class TrendingUserSerializer
        include FastJsonapi::ObjectSerializer
        set_type :user
        attribute :id, :nickname, :karma, :picture
    end

    class TrendingListsSerializer
        include FastJsonapi::ObjectSerializer
        set_type :list
        belongs_to :user, serializer: Discoveries::TrendingUserSerializer

        attributes :created_at
    end

    class TrendingSerializer
        include FastJsonapi::ObjectSerializer
        set_type :content
        has_many :lists, serializer: Discoveries::TrendingListsSerializer 
        attributes :url, :title, :created_at
end