CollectionProxy vs AssociationRelation [英] CollectionProxy vs AssociationRelation

问题描述

我想知道 ActiveRecord::Associations::CollectionProxy 和 ActiveRecord::AssociationRelation 之间的区别.

I am wondering about the difference between ActiveRecord::Associations::CollectionProxy and ActiveRecord::AssociationRelation.

class Vehicle < ActiveRecord::Base
  has_many :wheels
end

class Wheel < ActiveRecord::Base
  belongs_to :vehicle
end

如果我这样做:

v = Vehicle.new

v.wheels # =>#

v.wheels.all # =>#

我不知道它们之间有什么区别以及为什么以这种方式实现?

I have no idea what is the difference between them and why this is implemented this way?

推荐答案

ActiveRecord::Relation 在变成查询并执行之前是简单的查询对象，CollectionProxy 在另一只手稍微复杂一些.

ActiveRecord::Relation are simple query objects before being turned into a query and executed, CollectionProxy on the other hand are a little bit more sophisticated.

首先你会得到关联扩展，你可能看到过类似这样的东西，假设一个有很多书的书店模型

First of all you get association extentions, you probably saw something that looks like this, assume a book store model that has many books

class Store < ActiveRecord::Base
  has_many :books do
    def used
      where(is_used: true)
    end
  end
end

这样你就可以使用类似这样的语法调用商店中的旧书

This way you get to call used books in a store using a syntax that looks like this

Store.first.books.used

但这是最基本的用途，您可以使用集合代理中暴露给您的属性，它们是owner、reflection 和target

But this is the most basic uses, you could use the attributes that are exposed to you in the collection proxy, which are owner, reflection and target

owner 提供对持有关联的父对象的引用

The owner provides a a reference to the parent object holding the association

reflection 对象是 ActiveRecord::Reflection::AssocciationReflection 的一个实例，包含关联的所有配置选项.

The reflection object is an instance of ActiveRecord::Reflection::AssocciationReflection and contains all the configuration options for the association.

target 是关联集合对象(当has_one 和belongs_to 时为单个对象).

The target is the association collection objects (or single object when has_one and belongs_to).

使用这些方法，您可以在关联扩展中设置一些条件，例如，如果我们有一个博客，我们会将所有已删除帖子的访问权限授予管理员用户(我知道这是一个蹩脚的例子)

Using those methods you could do some conditons in your association extention, for example if we have a blog, we give access to all deleted posts to users who are admins (lame example I know)

Class Publisher < ActiveRecord::Base
  has_many :posts do
    def deleted
      if owner.admin?
        Post.where(deleted: true)
      else
        where(deleted: true)
      end
    end
  end
end

您还可以访问另外两个方法，它们是 reset 和 reload，第一个 (reset) 清除缓存的关联对象，第二个(reload)比较常用，用于reset然后从数据库中加载关联的对象.

You also get access to two more methods which are reset and reload, the first one (reset) clears the cached association objects, the second one (reload) is more common and is used to reset and then loads the associated objects from the database.

我希望这可以解释 CollectionProxy 类如何如此有用

I hope this explains how having a CollectionProxy class would be so useful

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