Ruby on rails Mongoid究竟何时向MongoDb发出查询

我很难找到清楚说明Mongoid在什么时候实际查询Mongo数据库的文档

假设我有一个名为Projects的模型，它引用了另一个名为Website的模型，如下所示

class Project
  include Mongoid::Document
  ...

  has_one :website
  ...

end

class Website
  include Mongoid::Document
  ...
  belongs_to :project
  ...

end

如果使用我的项目模型，我有许多方法可以访问网站对象的属性，我的问题是，对Mongodb的实际查询发生在哪一行：

class Project      
  ...
  def website_url
    @website ||= self.website #Does the query occur here?

    website.url # Or does is occur here?
  end

我可以预见，在请求对象的实际属性或属性之前推迟查询数据库是非常有益的。然而，我不知道如何测试这一点，以确定自己的答案

---

如果您将日志添加到应用程序中（Moped是Mongoid的底层Mongoid驱动程序），谢谢您

：

您将看到，根据配置的不同，被引用文档的属性检索可能会延迟到第一次需要时

例如，对于以下两个类：

class User
  include Mongoid::Document
  field :name, type: String

  belongs_to :band
end

class Band
  include Mongoid::Document
  field :name, type: String
  field :popularity, type: Integer

  has_many :users    
end

  MOPED: 127.0.0.1:27017 COMMAND      database=admin command={:ismaster=>1} (2.0020ms)
  MOPED: 127.0.0.1:27017 INSERT       database=mm collection=users documents=[{"_id"=>"528ebd08cbafedb879000001", "name"=>"Johnny"}] flags=[] (0.0000ms)
  MOPED: 127.0.0.1:27017 INSERT       database=mm collection=bands documents=[{"_id"=>"528ebd08cbafedb879000002", "name"=>"StackOverflowTo11", "popularity"=>10}] flags=[] (1.0011ms)
  MOPED: 127.0.0.1:27017 UPDATE       database=mm collection=users selector={"_id"=>"528ebd08cbafedb879000001"} update={"$set"=>{"band_id"=>"528ebd08cbafedb879000002"}} flags=[] (0.0000ms)
  MOPED: 127.0.0.1:27017 QUERY        database=mm collection=users selector={"$query"=>{"name"=>"Johnny"}, "$orderby"=>{:_id=>1}} flags=[:slave_ok] limit=-1 skip=0 batch_size=nil fields=nil (2.0020ms)
Just the user
  MOPED: 127.0.0.1:27017 QUERY        database=mm collection=bands selector={"$query"=>{"_id"=>"528eb8f3cbafed9682000002"}, "$orderby"=>{:_id=>1}} flags=[:slave_ok] limit=-1 skip=0 batch_size=nil fields=nil (16.0160ms)
StackOverflowTo11

然后像这样使用：

def find_user
    query = User.where(:name => 'Johnny')
    user = query.first  # just the first

    puts "Just the user"
    puts user.band.name
end

您可能会得到如下结果：

class User
  include Mongoid::Document
  field :name, type: String

  belongs_to :band
end

class Band
  include Mongoid::Document
  field :name, type: String
  field :popularity, type: Integer

  has_many :users    
end

  MOPED: 127.0.0.1:27017 COMMAND      database=admin command={:ismaster=>1} (2.0020ms)
  MOPED: 127.0.0.1:27017 INSERT       database=mm collection=users documents=[{"_id"=>"528ebd08cbafedb879000001", "name"=>"Johnny"}] flags=[] (0.0000ms)
  MOPED: 127.0.0.1:27017 INSERT       database=mm collection=bands documents=[{"_id"=>"528ebd08cbafedb879000002", "name"=>"StackOverflowTo11", "popularity"=>10}] flags=[] (1.0011ms)
  MOPED: 127.0.0.1:27017 UPDATE       database=mm collection=users selector={"_id"=>"528ebd08cbafedb879000001"} update={"$set"=>{"band_id"=>"528ebd08cbafedb879000002"}} flags=[] (0.0000ms)
  MOPED: 127.0.0.1:27017 QUERY        database=mm collection=users selector={"$query"=>{"name"=>"Johnny"}, "$orderby"=>{:_id=>1}} flags=[:slave_ok] limit=-1 skip=0 batch_size=nil fields=nil (2.0020ms)
Just the user
  MOPED: 127.0.0.1:27017 QUERY        database=mm collection=bands selector={"$query"=>{"_id"=>"528eb8f3cbafed9682000002"}, "$orderby"=>{:_id=>1}} flags=[:slave_ok] limit=-1 skip=0 batch_size=nil fields=nil (16.0160ms)
StackOverflowTo11

如果启用该功能并且

包含

函数调用，则会立即检索关系（或从缓存中检索）