mongoid4.0.0 去掉了 IdentityMap

原来在 mongoid3 上做的 belongs_to eager_load 不能再用了 那我们看看 mongoid4.0.0 改如何修改

模型，假设一张脸有多个左眼和多个右眼吧，懒得改了^^

class Face
  include Mongoid::Document
  field :name, type: String
  has_many :left_eyes, class_name: "Eye", as: :eyeable
  has_many :right_eyes, class_name: "Eye", as: :eyeable
end

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

  belongs_to :eyeable, polymorphic: true
end

no eager_load

faces = Face.all.to_a
faces.first.left_eyes

MOPED: 127.0.0.1:27017 QUERY runtime: 0.6560ms

[ #, # ]

has_many: eager_load , 使用 .includes 方法

faces = Face.includes(:left_eyes).to_a
faces.first.left_eyes

[ #, # ]

mongoid 4.0.0 支持 has_many 的 eager_load，但是不支持 belongs_to

eg:

eyes = Eye.includes(:eyeable).to_a

抛出异常 Mongoid::Errors::EagerLoad: Problem: Eager loading :eyeable is not supported since it is a polymorphic belongs_to relation.

查看原代码，发现当 belongs_to 后面接 polymorphic 参数的时候，会抛出异常

module Mongoid
  module Relations
    module Eager
      class BelongsTo < Base
        def preload
           raise Errors::EagerLoad.new(@metadata.name) if @metadata.polymorphic?
           @docs.each do |d|
             set_relation(d, nil)
           end

           each_loaded_document do |doc|
             id = doc.send(key)
             set_on_parent(id, doc)
           end
           ...
        end
      end
    end
  end
end

那我们注意掉这句，看一下会发生什么？ NameError: uninitialized constant Eyeable 问题出在这里 each_loaded_document, @metadata.klass ＝> Eyeable 这个 Eyeable 是从哪里来的呢？就是从我们的模型 Eye 中：belongs_to :eyeable each_loaded_document 这个方法就是 load 依赖的模型数据了

eyes = Eye.includes(:eyeable).to_a

module Mongoid
  module Relations
    module Eager
      class Base
        def each_loaded_document
          @metadata.klass.any_in(key => keys_from_docs).each do |doc|
            yield doc
          end
        end
      end
    end
  end
end

找到需要预加载的 Face，并通过 set_on_parent 与 eye 建立对应关系 @metadata.klass.any_in(key => keys_from_docs) => Face.any_in(_id => ['id1','id2'])

each_loaded_document do |doc|
   id = doc.send(key)
   set_on_parent(id, doc)
end

建立 eye 对象和 face 对象的对应关系 #Eye:0x007f837b55b670, #Face:0x007f837b4f2800 #Eye:0x007f837b55b5f8, #Face:0x007f837b4f2800

def set_on_parent(id, element)
  grouped_docs[id].each do |d|
    set_relation(d, element)
  end
end

生成实例变量 @_eyealbe,当调用 eyes.first.eyealbe 的时候，则直接返回 face 对象 而不需要再查询数据库 @_eyeable, #Face:0x007f837b4f2800

def set_relation(name, relation)
  instance_variable_set("@_#{name}", relation)
end

综上，处理 belongs_to eager_load 的做法：

1. 重写 preload 方法，去掉 raise lib/mongoid/eager_load.rb

   module Mongoid
   module Relations
   module Eager
     class BelongsTo < Base
       def preload
          @docs.each do |d|
            set_relation(d, nil)
          end
   
          each_loaded_document do |doc|
            id = doc.send(key)
            set_on_parent(id, doc)
          end
          ...
       end
     end
   end
   end
   end
   

2. 加载 initializers/mongoid.rb

   require "mongoid/eager_load"
   

3. 在 belongs_to 一端，声明 class_name

   class Eye
   include Mongoid::Document
   field :name, type: String
   belongs_to :eyeable, class_name: "Face", polymorphic: true
   end
   

测试：belongs_to

2.1.1 :004 > eyes = Eye.all.to_a 2.1.1 :005 > eyes.first.eyeable MOPED: 127.0.0.1:27017 QUERY runtime: 0.6910ms => #

2.1.1 :002 > eyes = Eye.includes(:eyeable).to_a 2.1.1 :003 > eyes.first.eyeable => #

测试：has_many

2.1.1 :006 > faces = Face.includes(:left_eyes).to_a 2.1.1 :009 > faces.first.left_eyes MOPED: 127.0.0.1:27017 QUERY runtime: 0.7480ms => [ #, # ]

2.1.1 :006 > faces = Face.includes(:left_eyes).to_a 2.1.1 :007 > faces.first.left_eyes => [ #, # ]

博客：http://michael-roshen.iteye.com/blog/2152764 微信：ruby 程序员