如何在ActiveRecord中为Post集合关联查询Account对象？

背景与问题描述 #

现有模型关系：Organization与Post为一对多（多态关联），Post属于Account。模型定义如下：

# Post class
class Post < ApplicationRecord
  belongs_to :account, foreign_key: 'account_id'
  belongs_to :postable, polymorphic: true

  validates :content, length: { in: 1..500 }
end

# Organization
class Organization < ApplicationRecord
  has_many :posts, as: :postable
end

通过Organization.first.posts获取组织帖子时，返回的JSON结构仅包含accountId，不包含完整Account对象：

[
    {
        "id": 101,
        "accountId": 50,
        "postableType": "Organization",
        "postableId": 3,
        "content": "Consequatur in illum vel voluptates.",
        "createdAt": "2022-11-23T04:57:45.271Z",
        "updatedAt": "2022-11-23T04:57:45.271Z"
    },
    {
        "id": 102,
        "accountId": 46,
        "postableType": "Organization",
        "postableId": 3,
        "content": "Fugit totam minus et consequatur.",
        "createdAt": "2022-11-23T04:57:45.274Z",
        "updatedAt": "2022-11-23T04:57:45.274Z"
    }
]

期望每个Post对象中包含对应的Account对象，理想结构如下：

[
    {
        "id": 101,
        "account": {
            "id": 46,
            "firstName": "Bob"
        },
        "postableType": "Organization",
        "postableId": 3,
        "content": "Consequatur in illum vel voluptates.",
        "createdAt": "2022-11-23T04:57:45.271Z",
        "updatedAt": "2022-11-23T04:57:45.271Z"
    },
    {
        "id": 102,
        "account": {
            "id": 46,
            "firstName": "Bob"
        },
        "postableType": "Organization",
        "postableId": 3,
        "content": "Fugit totam minus et consequatur.",
        "createdAt": "2022-11-23T04:57:45.274Z",
        "updatedAt": "2022-11-23T04:57:45.274Z"
    }
]

尝试过Organization.first.posts.includes(:account)和Organization.first.posts.joins(:account)，但返回结果未变化，询问实现需求的最佳方式。

解决方案 #

1. 序列化层显式包含关联数据（推荐） #

includes(:account)只是预加载关联数据避免N+1查询，但不会自动将Account对象加入JSON输出，需要在序列化配置中指定：

场景1：使用Jbuilder

在对应的Jbuilder模板中，显式渲染account属性：

# app/views/posts/index.json.jbuilder
json.array! @posts do |post|
  json.extract! post, :id, :postable_type, :postable_id, :content, :created_at, :updated_at
  json.account do
    json.extract! post.account, :id, :first_name
  end
end

场景2：使用ActiveModel::Serializers

创建PostSerializer并指定包含account关联：

# app/serializers/post_serializer.rb
class PostSerializer < ActiveModel::Serializer
  attributes :id, :postable_type, :postable_id, :content, :created_at, :updated_at
  belongs_to :account
end

# 对应的AccountSerializer（按需定义字段）
class AccountSerializer < ActiveModel::Serializer
  attributes :id, :first_name
end

在控制器中调用序列化：

def index
  organization = Organization.first
  @posts = organization.posts.includes(:account)
  render json: @posts, serializer: PostSerializer, include: :account
end

2. 手动构建哈希（无序列化器时） #

如果不想使用序列化库，可以手动构建包含Account的哈希数组，结合includes(:account)避免N+1：

organization = Organization.first
posts = organization.posts.includes(:account).map do |post|
  post.as_json(
    only: [:id, :postable_type, :postable_id, :content, :created_at, :updated_at],
    include: {
      account: { only: [:id, :first_name] }
    }
  )
end

# 此时posts就是包含account的哈希数组，可直接返回
render json: posts

3. 避免遍历单独查询（不推荐） #

不要遍历每个Post单独查询Account，这会导致N+1查询问题（每个Post触发一次Account查询），严重影响性能。includes(:account)已经能批量预加载所有关联的Account，只需确保序列化时包含即可。

内容的提问来源于stack exchange，提问作者DrummerGenius