ActiveRecord :: Relation对象如何调用类方法 [英] How can an ActiveRecord::Relation object call class methods

问题描述

ActiveRecord :: Relation对象如何调用类方法？

How can an ActiveRecord::Relation object call class methods?

class Project < ActiveRecord::Base
  has_many :tasks
end

class Task < ActiveRecord::Base
  belongs_to :project

  def self.initial_tasks # class methods
   # here return initial tasks
  end
end

现在我们可以调用：

Project.first.tasks.initial_tasks # how it works

initial_tasks 是类方法，我们不能在对象上调用类方法。

initial_tasks is a class method and we can't call class methods on an object.

Project.first.tasks 返回一个ActiveRecord :: Relation对象，那么它怎么能够调用 initial_tasks ？

Project.first.tasks returns an ActiveRecord::Relation object, so how could it be able to call initial_tasks?

请解释。

推荐答案

关于应用程序中关于 ActiveRecord的类方法的文档并不多：关系对象，但我们可以通过查看 ActiveRecord作用域工作。

There's not much documentation on the application on class methods to ActiveRecord::Relation objects, but we can understand this behavior by taking a look at how ActiveRecord scopes work.

首先，Rails模型作用域将返回 ActiveRecord :: Relation 对象。从文档中：

First, a Rails model scope will return an ActiveRecord::Relation object. From the docs:

模型上的类方法可自动在范围内使用。假设进行以下设置：

Class methods on your model are automatically available on scopes. Assuming the following setup:

class Article < ActiveRecord::Base
  scope :published, -> { where(published: true) }
  scope :featured, -> { where(featured: true) }

  def self.latest_article
    order('published_at desc').first
  end

  def self.titles
    pluck(:title)
  end
end

首先，调用范围将返回 ActiveRecord :: Relation 对象：

First, invoking a scope returns an ActiveRecord::Relation object:

Article.published.class
#=> ActiveRecord::Relation

Article.featured.class
#=> ActiveRecord::Relation

然后，您可以在 ActiveRecord :: Relation上进行操作对象，使用相应模型的类方法：

Then, you can operate on the ActiveRecord::Relation object using the respective model's class methods:

Article.published.featured.latest_article
Article.featured.titles

了解类方法和 ActiveRecord :: Relation ，但要旨是：

It's a bit of a roundabout way of understanding the relationship between class methods and ActiveRecord::Relation, but the gist is this:

1. 按定义，模型范围返回 ActiveRecord :: Relation 对象


2. 通过定义，范围可以访问类方法


3. 因此 ， ActiveRecord :: Relation 对象具有访问权限类方法

1. By definition, model scopes return ActiveRecord::Relation objects
2. By definition, scopes have access to class methods
3. Therefore, ActiveRecord::Relation objects have access to class methods

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