Question

我的应用中有很多案例，用户在与另一个对象（“组”）的关联中只有一个对象（比如“描述”）。

例如：

class User < ActiveRecord::Base
  has_many :descriptions
  has_many :groups

class Group < ActiveRecord::Base
  has_many :users
  has_many :descriptions

class Description < ActiveRecord::Base
  belongs_to :user
  belongs_to :group

如果我想渲染特定群组中的所有用户并包含相关说明，我可以做一些事情：

#users model
def description_for(group_id)
  descriptions.find_by_group_id(group_id)
end

#view
@group.users.each do |user|
   user.name
   user.description_for(@group.id).content

但是这会产生大量的描述查询。我尝试过使用连接：

#controller
@group = Group.find(params[:id], :joins => [{:users => :descriptions}], :conditions => ["descriptions.group_id = ?", params[:id]])

但是因为我仍在调用user.description_for（@ group.id），所以它对页面加载没有帮助。

更新：示例生成的SQL

Rendered users/_title.html.haml (1.6ms)
CACHE (0.0ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 37 LIMIT 1
User Load (0.2ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 7 LIMIT 1
CACHE (0.0ms)  SELECT "groups".* FROM "groups" WHERE "groups"."id" = 28 LIMIT 1
Description Load (0.1ms)  SELECT "descriptions".* FROM "descriptions" WHERE "descriptions"."target_id" = 7 AND "descriptions"."group_id" = 28 LIMIT 1
CACHE (0.0ms)  SELECT "descriptions".* FROM "descriptions" WHERE "descriptions"."target_id" = 7 AND "descriptions"."group_id" = 28 LIMIT 1
Rendered users/_title.html.haml (1.7ms)
CACHE (0.0ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 37 LIMIT 1
User Load (0.3ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 51 LIMIT 1
CACHE (0.0ms)  SELECT "groups".* FROM "groups" WHERE "groups"."id" = 28 LIMIT 1
Description Load (0.1ms)  SELECT "descriptions".* FROM "descriptions" WHERE "descriptions"."target_id" = 51 AND "descriptions"."group_id" = 28 LIMIT 1
CACHE (0.0ms)  SELECT "descriptions".* FROM "descriptions" WHERE "descriptions"."target_id" = 51 AND "descriptions"."group_id" = 28 LIMIT 1
Rendered users/_title.html.haml (1.8ms)
CACHE (0.0ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 37 LIMIT 1
User Load (0.2ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 5 LIMIT 1
CACHE (0.0ms)  SELECT "groups".* FROM "groups" WHERE "groups"."id" = 28 LIMIT 1
Description Load (0.1ms)  SELECT "descriptions".* FROM "descriptions" WHERE "descriptions"."target_id" = 5 AND "descriptions"."group_id" = 28 LIMIT 1
CACHE (0.0ms)  SELECT "descriptions".* FROM "descriptions" WHERE "descriptions"."target_id" = 5 AND "descriptions"."group_id" = 28 LIMIT 1
Rendered users/_title.html.haml (1.7ms)
CACHE (0.0ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 37 LIMIT 1
User Load (0.2ms)  SELECT "users".* FROM "users" WHERE "users"."id" = 52 LIMIT 1
CACHE (0.0ms)  SELECT "groups".* FROM "groups" WHERE "groups"."id" = 28 LIMIT 1
Description Load (0.2ms)  SELECT "descriptions".* FROM "descriptions" WHERE "descriptions"."target_id" = 52 AND "descriptions"."group_id" = 28 LIMIT 1
CACHE (0.0ms)  SELECT "descriptions".* FROM "descriptions" WHERE "descriptions"."target_id" = 52 AND "descriptions"."group_id" = 28 LIMIT 1
Rendered users/_title.html.haml (1.7ms)

Answer 1

是的，我认为实际上你不需要rails 3中的joins子句。如果你使用include和where，Arel会为你做艰苦的工作。

我已经测试了这个（尽管使用了一组不同于你的模型（和属性））使用具有相同基本排列关联的模型，我认为这应该有效：

在models / user.rb中：

scope :with_group_and descriptions, lambda { |group_id| includes(:groups, :descriptions).where(:groups => { :id => group_id }, :descriptions => { :group_id => group_id }) }

然后在你的控制器中你打电话：

@users = User.with_group_and_descriptions(params[:id])

最后在视图中，您可以这样做：

@users.each do |user|
  user.name
  user.descriptions.each do |desc|
    desc.content
# or
@users.each do |user|
  user.name
  user.descriptions[0].content

如果我的想法正确，那么这应该只进行2个db调用。一个用于获取user_ids列表，另一个用于获取用户，组和描述数据，即使您正在调用用户对象的描述方法，该方法通常包含所有描述（不仅仅是特定组的描述）），因为你已经填充了关联栏，当你拨打user.descriptions时，栏杆不会再次抓取所有关联，而是只使用描述列出你从数据库中提取的那些。 group_id where子句。但是，调用user.descriptions(true)将强制重新加载描述，从而返回一个所有用户的描述关联数组。

Answer 2

看一下include - 它指定了一个应该加载的关联。

Railscasts #181: Include vs Joins（或ASCIIcast version）
Ruby on Rails Guides - 请参阅第4.1.2.7节

Rails 3 Eager正在加载条件 - 如何访问预先加载的数据？

2 个答案: