nested query on active model serializer attribute

Question

I have two models, Artwork and Event, an artwork has many events. Some events are confirmed.

In my serializer I want to return the last confirmed event for each Artwork

Right now it does but also creates a slow (n+1 ?) query where each serialized Artwork fetches all events.

How can I do something faster ?

# artwork_controller.rb
      def index
        @artworks = Artwork.all
        render json: @artworks
      end

# artwork_serializer.rb
      attributes :id, :name, :photo, :created_at

      has_one :event do
        object.events.confirmed.last
      end

and logs

Processing by Api::V1::ArtworksController#index as JSON
  Passenger Load (2.0ms)  SELECT "artworks".* FROM "artworks"
  ↳ app/controllers/api/v1/artworks_controller.rb:11
[active_model_serializers]   Event Load (1.2ms)  SELECT  "events".* FROM "events" WHERE "events"."artwork_id" = $1 AND "events"."published" = $2 ORDER BY "events"."created_at" DESC LIMIT $3  [["artwork_id", 16], ["published", true], ["LIMIT", 1]]
[active_model_serializers]   ↳ app/serializers/api/v1/artwork_serializer.rb:9
[active_model_serializers]   Event Load (0.8ms)  SELECT  "events".* FROM "events" WHERE "events"."artwork_id" = $1 AND "events"."published" = $2 ORDER BY "events"."created_at" DESC LIMIT $3  [["artwork_id", 9], ["published", true], ["LIMIT", 1]]
[active_model_serializers]   ↳ app/serializers/api/v1/artwork_serializer.rb:9
[active_model_serializers]   Event Load (2.1ms)  SELECT  "events".* FROM "events" WHERE "events"."artwork_id" = $1 AND "events"."published" = $2 ORDER BY "events"."created_at" DESC LIMIT $3  [["artwork_id", 27], ["published", true], ["LIMIT", 1]]
[active_model_serializers]   ↳ app/serializers/api/v1/artwork_serializer.rb:9#
# ... for each artwork

[active_model_serializers]   ↳ app/serializers/api/v1/artwork_serializer.rb:9
[active_model_serializers]   User Load (1.7ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 84], ["LIMIT", 1]]
[active_model_serializers]   ↳ app/serializers/api/v1/event_serializer.rb:9
[active_model_serializers]   CACHE Event Load (0.0ms)  SELECT  "events".* FROM "events" WHERE "events"."artwork_id" = $1 AND "events"."published" = $2 ORDER BY "events"."created_at" DESC LIMIT $3  [["artwork_id", 9], ["published", true], ["LIMIT", 1]]
[active_model_serializers]   ↳ app/serializers/api/v1/artwork_serializer.rb:9
[active_model_serializers]   User Load (0.6ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 88], ["LIMIT", 1]]
[active_model_serializers]   ↳ app/serializers/api/v1/event_serializer.rb:9
[active_model_serializers]   CACHE Event Load (0.0ms)  SELECT  "events".* FROM "events" WHERE "events"."artwork_id" = $1 AND "events"."published" = $2 ORDER BY "events"."created_at" DESC LIMIT $3  [["artwork_id", 27], ["published", true], ["LIMIT", 1]]
[active_model_serializers]   ↳ app/serializers/api/v1/artwork_serializer.rb:9
[active_model_serializers]   User Load (0.5ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 LIMIT $2  [["id", 76], ["LIMIT", 1]]
[active_model_serializers]   ↳ app/serializers/api/v1/event_serializer.rb:9
[active_model_serializers]   CACHE Event Load (0.0ms)  SELECT  "events".* FROM "events" WHERE "events"."artwork_id" = $1 AND "events"."published" = $2 ORDER BY "events"."created_at" DESC LIMIT $3  [["artwork_id", 11], ["published", true], ["LIMIT", 1]]
# ... for each artwork events

Answer 1

you can try this:

Please add this association to the artwork model.

# app/models/artwork.rb

has_one :last_confirmed_event, -> { order(created_at: :desc).where(confirmed: true) }, class_name: "Event"

In your artworks controller add eager-loading.

# app/controllers/artwork_controllers.rb

def index 
  @artworks = Artwork.all.includes(:last_confirmed_event)
  render json: @artworks 
end

At the last use declared association in your serializer.

# app/serializers/artwork_serializer.rb

attributes :id, :name, :photo, :created_at

has_one :last_confirmed_event

You can change the association name as per your choice.