Active Model Serializer caching of calculated field - ruby-on-rails

Question

I'm trying to to cache a collection of items, this is my AMS:
class ApplicationCategorySerializer < ActiveModel::Serializer
cache({})
cache key: 'application_category', expires_in: 3.days
attributes :id
attributes :name
attributes :active
attributes :group
attributes :num_of_protocols
belongs_to :group do |serializer|
serializer.attributes[:group][:name]
end
def num_of_protocols
ApplicationCategory.find(object.id).protocol_categories.count
end
end
My problem is that without the :num_of_protocols part it takes 10 times faster to generate the response,
when I look at the log I can see:
Started GET "/application_categories.json" for 127.0.0.1 at 2016-08-04 19:12:27 +0300
Processing by ApplicationCategoriesController#index as JSON
ApplicationCategory Load (3.0ms) SELECT "application_categories".* FROM "application_categories"
[active_model_serializers] Group Load (1.0ms) SELECT "groups".* FROM "groups" WHERE "groups"."id" = ? LIMIT 1 [["id", 2066]]
[active_model_serializers] ApplicationCategory Load (0.0ms) SELECT "application_categories".* FROM "application_categories" WHERE "application_categories"."id" = ? LIMIT 1 [["id", 1]]
[active_model_serializers] (0.0ms) SELECT COUNT(*) FROM "protocol_categories" INNER JOIN "application_categories_protocol_categories" ON "protocol_categories"."id" = "application_categories_protocol_categories"."protocol_category_id" WHERE "application_categories_protocol_categories"."application_category_id" = ? [["application_category_id", 1]]
[active_model_serializers] CACHE (0.0ms) SELECT "groups".* FROM "groups" WHERE "groups"."id" = ? LIMIT 1 [["id", 2066]]
[active_model_serializers] ApplicationCategory Load (0.0ms) SELECT "application_categories".* FROM "application_categories" WHERE "application_categories"."id" = ? LIMIT 1 [["id", 2]]
[active_model_serializers] (0.0ms) SELECT COUNT(*) FROM "protocol_categories" INNER JOIN "application_categories_protocol_categories" ON "protocol_categories"."id" = "application_categories_protocol_categories"."protocol_category_id" WHERE "application_categories_protocol_categories"."application_category_id" = ? [["application_category_id", 2]]
[active_model_serializers] CACHE (0.0ms) SELECT "groups".* FROM "groups" WHERE "groups"."id" = ? LIMIT 1 [["id", 2066]]
...................... [many many more lines like those...]
[active_model_serializers] CACHE (0.0ms) SELECT "application_categories".* FROM "application_categories" WHERE "application_categories"."id" = ? LIMIT 1 [["id", 608]]
[active_model_serializers] CACHE (0.0ms) SELECT COUNT(*) FROM "protocol_categories" INNER JOIN "application_categories_protocol_categories" ON "protocol_categories"."id" = "application_categories_protocol_categories"."protocol_category_id" WHERE "application_categories_protocol_categories"."application_category_id" = ? [["application_category_id", 608]]
[active_model_serializers] CACHE (0.0ms) SELECT "application_categories".* FROM "application_categories" WHERE "application_categories"."id" = ? LIMIT 1 [["id", 609]]
[active_model_serializers] CACHE (0.0ms) SELECT COUNT(*) FROM "protocol_categories" INNER JOIN "application_categories_protocol_categories" ON "protocol_categories"."id" = "application_categories_protocol_categories"."protocol_category_id" WHERE "application_categories_protocol_categories"."application_category_id" = ? [["application_category_id", 609]]
[active_model_serializers] CACHE (0.0ms) SELECT "application_categories".* FROM "application_categories" WHERE "application_categories"."id" = ? LIMIT 1 [["id", 610]]
[active_model_serializers] CACHE (0.0ms) SELECT COUNT(*) FROM "protocol_categories" INNER JOIN "application_categories_protocol_categories" ON "protocol_categories"."id" = "application_categories_protocol_categories"."protocol_category_id" WHERE "application_categories_protocol_categories"."application_category_id" = ? [["application_category_id", 610]]
[active_model_serializers] Rendered ActiveModel::Serializer::CollectionSerializer with ActiveModelSerializers::Adapter::Attributes (1521.15ms)
how can I get better results with caching? - when I comment out the cache lines I get aprox. same times as with them, when I remove the :num_of_protocols I get same time with & without cache.
what am I configuring wrong?

Answer 1

Can you do this?
def num_of_protocols
object.protocol_categories.count
end
And, it's been a while for serializers for me, but I thought you were able to do this...
def num_of_protocols
protocol_categories.count
end
And this is better ruby (in the case protocal_categories is already loaded as an array)
def num_of_protocols
protocol_categories.size
end
And, if all that works or not, make sure you include protocol_categories and groups when you instantiate #application_category before you render to json. You're doing too many queries.