How to render a part of fields embed model?

Question

I want to render a part of embed model's fields.

I wrote bellow code. But, I got a response include all embed model's fields.

class RacesController < ApplicationController
  def index
    render json: Race.all, include: 'held.held_year', status: :ok
  end
end

class RaceSerializer < ActiveModel::Serializer
  attributes :id, :held_id, :course_id, :race_round, :name, :created_at, :updated_at

  belongs_to :held
end

class HeldSerializer < ActiveModel::Serializer
  attributes :id, :racecourse_id, :held_year, :held_month, :held_day,
             :number_of_times, :number_of_days, :created_at, :updated_at

  has_many :races
end

Sent Request.

GET /races

Gotten response.

[
  {
    "id": 7,
    "held_id": 1,
    "course_id": 2,
    "race_round": 7,
    "name": "３歳１勝クラス",
    "created_at": "2022-04-13T07:49:30.000Z",
    "updated_at": "2022-04-13T07:49:30.000Z",
    "held": {
      "id": 1,
      "racecourse_id": 6,
      "held_year": 22,
      "held_month": 4,
      "held_day": 9,
      "number_of_times": 3,
      "number_of_days": 5,
      "created_at": "2022-04-13T07:49:30.000Z",
      "updated_at": "2022-04-13T07:49:30.000Z"
    }
  },...
]

I expect the response only include held.held_year . Expected response is bellow.

[
  {
    "id": 7,
    "held_id": 1,
    "course_id": 2,
    "race_round": 7,
    "name": "３歳１勝クラス",
    "created_at": "2022-04-13T07:49:30.000Z",
    "updated_at": "2022-04-13T07:49:30.000Z",
    "held": {
      "held_year": 22
    }
  },...
]

How Should I fix the code?

Answer 1

I think you can add only: param to your render part. Something like this:

render json: Race.includes(:held), include: { held: { only: :held_year } }, status: :ok

Note Race.includes(:held) to avoid N+1.