Rails 3 带条件的预加载 - 如何访问预加载的数据？

Question

我的应用程序中有很多情况，用户在与另一个对象（“组”）的关联中只有一个对象（例如“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

如果我想呈现某个组中的所有用户并包含他们的相关描述，我可以这样做：

#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

但这会生成大量的描述查询。我尝试过使用连接：

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

但由于我仍在调用 user.description_for(@group.id) 它对页面加载没有帮助。

更新：生成的 SQL 示例

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

看一下 include——它指定了一个应该立即加载的关联。

• Railscasts #181：包含与连接（或 ASCIIcast 版本)
• Ruby on Rails 指南 - 请参阅第 4.1.2.7 节

Answer 2

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会为你做这些艰苦的工作。

我已经使用具有相同底层关联安排的模型对此进行了测试（尽管使用了与您不同的一组模型（和属性）），并且我认为这应该有效：

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 }) }

然后在您的控制器中调用：

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

最后在视图中你可以这样做：

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

如果我的想法是正确的，那么这只应该进行 2 次 db 调用。第一个用于获取 user_ids 列表，第二个用于获取用户、组和描述数据，即使您正在调用用户对象的描述方法，该方法通常会包含所有描述（而不仅仅是特定组的描述） ），因为您已经填充了关联栏，当您调用 user.descriptions 时，不会再次抓取所有关联，而是仅列出您从数据库使用descriptions.group_id where 子句。然而，调用user.descriptions(true)将强制重新加载描述，从而导致它返回用户的所有描述关联的数组。