优雅的高级 Rails 查询

Question

我无法理解更高级的 Rails 查询方法。在本例中，我有三个表：Users、Friendships 和 Events。

我在每个模型中都设置如下

用户-> :has_many => :friendships, :has_many => :items

友谊-> :belongs_to => :user

事件-> belongs_to => :user

友谊只是两列：“user_id”和“friend_id”，因此每个新友谊都会创建两个新行。

这是我用来查找“属于 current_user 的朋友的前四个事件，其 start_date 晚于现在”的查询。

find_by_sql(["SELECT DISTINCT e.id
              FROM events e
              WHERE e.start_date > ?
              AND e.user_id IN(
              SELECT f.user_id
              FROM friendships f
              WHERE f.friend_id = ?)
              LIMIT 0,4", Time.zone.now,current_user.id])

在 Rails 中做这种事情的真正优雅的方法是什么？我感觉这也太低效了……

Answer 1

您应该可以使用:join 和:conditions

time_range = (Time.now.midnight - 4.days)..Time.now.midnight
Event.all :joins => :friendships, :conditions => 
   {'start_date' => time_range, 'friendships.friend_id' => current_user.id}, :limit => 4

我还没有真正完成很多这样的复杂查询，但我从这里的示例中拼凑起来：http://guides.rubyonrails.org/active_record_querying.html

编辑： 您可能需要添加到事件：

has_many :friends, :through => :friendships

Edit2：看起来你必须实际使用嵌套连接：

Event.all :joins => {:user => :friendships }, :conditions => {'start_date' => time_range, 'friendships.friend_id' => current_user.id }

这是我使用的模型代码：

class User < ActiveRecord::Base
  has_many :events, :foreign_key => "user_id"
  has_many :friendships
  has_many :friends, :through => :friendships, :source => :user, :uniq => true
end

class Event < ActiveRecord::Base
  belongs_to :user
  has_many :friendships, :through => :user
end

class Friendship < ActiveRecord::Base
  belongs_to :user
end

并且，如果可能有帮助的话，一些示例输出：

>> Event.all :joins => {:user => :friendships }, :conditions => {'happens_on' => time_range, 'friendships.user_id' => 1 }
Event.all :joins => {:user => :friendships }, :conditions => {'happens_on' => time_range, 'friendships.user_id' => 1 }
=> [#<Event id: 1, happens_on: "2010-08-06 00:42:37", title: "Jims party", description: "Happy Birthday", created_at: "2010-08-03 00:42:37", updated_at: "2010-08-03 00:42:37", user_id: 1>]
>> jim = User.find(1)
jim = User.find(1)
=> #<User id: 1, name: "Jim", bio: "Loves Rails", age: 30, created_at: "2010-08-03 00:30:51", updated_at: "2010-08-03 00:30:51">
>> crystal = User.find(2)
crystal = User.find(2)
=> #<User id: 2, name: "Crystal", bio: "Loves writing", age: 26, created_at: "2010-08-03 00:31:14", updated_at: "2010-08-03 00:31:14">
>> jim.events
jim.events
=> [#<Event id: 1, happens_on: "2010-08-06 00:42:37", title: "Jims party", description: "Happy Birthday", created_at: "2010-08-03 00:42:37", updated_at: "2010-08-03 00:42:37", user_id: 1>]
>> event1 = jim.events[0]
event1 = jim.events[0]
=> #<Event id: 1, happens_on: "2010-08-06 00:42:37", title: "Jims party", description: "Happy Birthday", created_at: "2010-08-03 00:42:37", updated_at: "2010-08-03 00:42:37", user_id: 1>
>> event1.user
event1.user
=> #<User id: 1, name: "Jim", bio: "Loves Rails", age: 30, created_at: "2010-08-03 00:30:51", updated_at: "2010-08-03 00:30:51">
>> event1.friendships
event1.friendships
=> [#<Friendship user_id: 1, friend_id: nil, created_at: "2010-08-03 00:57:31", updated_at: "2010-08-03 00:57:31">]
>>