ruby-on-rails - Rails 3 Eager Loading with conditions - 如何访问急切加载的数据？

Question

我的应用程序中有很多情况，其中用户在与另一个对象(“组”)的关联中不超过一个对象(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

但这会产生大量的描述查询。我试过使用连接:

#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: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，我认为实际上您不需要 rails 3 中的 joins 子句。如果您使用 include 和 where，Arel 将为您完成艰苦的工作。

我已经使用具有相同底层关联排列的模型对此进行了测试(尽管使用了与您的模型(和属性)不同的模型集)，我认为这应该可行:

在模型/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 Controller 中调用:

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

最后在 View 中，您可以执行以下操作:

@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 次分贝调用。一个获取 user_ids 列表，第二个获取用户、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) )，因为您已经填充了关联 rails ，所以当您调用 user.descriptions 时，不会再次抓取所有关联。 ，相反，它只会列出您使用 descriptions.group_id where 子句从数据库中提取的那些。调用 user.descriptions(true)但是将强制重新加载导致它返回数组 的描述。全部 用户的描述关联。