如何在Rails中将参数传递给委托方法 [英] How to pass argument to delegate method in Rails

问题描述

我希望有一个仪表板来显示多个模型的摘要，并且我使用Presenter来实现它而没有自己的数据.我使用ActiveModel类(没有数据表):

I would like to have a Dashboard to display summary of multiple models, and I implemented it using Presenter without its own data. I use an ActiveModel class (without data table):

class Dashboard
  attr_accessor :user_id
  def initialize(id)
    self.user_id = id
  end

  delegate :username, :password, :to => :user 
  delegate :address,  :to => :account
  delegate :friends,   :to => :friendship

end 

通过代理，我希望能够呼叫Dashboard.address并取回Account.find_by_user_id(Dashboard.user_id).address.

By delegate, I want to be able to call Dashboard.address and get back Account.find_by_user_id(Dashboard.user_id).address.

如果Dashboard是ActiveRecord类，那么我可以声明Dashboard#belongs_to :account，并且委托将自动运行(即Account知道它应该从Dashboard实例中的user_id等于to user_id的帐户返回地址属性).

If Dashboard was an ActiveRecord class, then I could have declared Dashboard#belongs_to :account and delegate would work automatically (i.e., Account would know it should return address attribute from account with user_id equals to user_id in Dashboard instance).

但是Dashboard不是ActiveRecord类，因此我无法声明belongs_to.我需要另一种方法来告诉Account查找正确的记录.

But Dashboard is not an ActiveRecord class, so I can't declare belongs_to. I need another way to tell Account to lookup the right record.

有没有办法解决这个问题? (我知道我可以伪造Dashboard来拥有一个空表，或者我可以将User的实例方法重写为带有参数的类方法.但是这些解决方案都是黑客.)

Is there a way to overcome this problem? (I know I can fake Dashboard to have an empty table, or I can rewrite User's instance methods to class methods that take argument. But these solutions are all hacks).

谢谢.

推荐答案

编写delegate :address, :to => :account时，这将在Dashboard上创建一个新的address方法，该方法基本上在同一对象上调用account方法，然后调用<此account方法的结果上的c7>.这(大致)类似于写作:

When you write delegate :address, :to => :account, this creates a new address method on Dashboard which basically calls the account method on the same object and then calls address on the result of this account method. This is (very roughly) akin to writing:

class Dashboard
 ...
  def address
    self.account.address
  end
 ...
end

对于当前的类，您要做的就是创建一个account方法，该方法返回具有正确的user_id的帐户:

With your current class, all you have to do is to create an account method which returns the account with the correct user_id:

class Dashboard
  attr_accessor :user_id
  def initialize(id)
    self.user_id = id
  end

  delegate :username, :password, :to => :user 
  delegate :address,  :to => :account
  delegate :friends,   :to => :friendship

  def account
    @account ||= Account.find_by_user_id(self.user_id)
  end
end

这将允许您访问如下地址:

This would allow you to access the address like this:

dashboard = Dashboard.new(1)
# the following returns Account.find_by_user_id(1).address
address = dashboard.address

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