Rails has_one : 通过关联 [英] Rails has_one :through association

问题描述

Rails 有一个 has_one :through 关联，它帮助通过第二个模型与第三个模型建立一对一的关联.除了建立快捷方式关联之外，它的真正用途是什么，否则会多走一步.

Rails has a has_one :through association that helps set up a one-to-one association with a third model by going through a second model. What is the real use of that besides making a shortcut association, that would otherwise be an extra step away.

以 Rails 指南为例:

Taking this example from the Rails guide:

class Supplier < ActiveRecord::Base
  has_one :account
  has_one :account_history, :through => :account
end

class Account < ActiveRecord::Base
  belongs_to :supplier
  has_one :account_history
end

class AccountHistory < ActiveRecord::Base
  belongs_to :account
end

可能允许我们做类似的事情:

might allow us to do something like:

supplier.account_history

否则将达到:

supplier.account.history

如果只是为了更简单的访问，那么从技术上讲，可能存在一对一的关联，将模型与通过 n-1 个模型的某个第 n 个模型连接起来，以便于访问.除了快捷方式之外，我还缺少其他什么吗?

If it's only for simpler access then technically there could be a one-to-one association that connects a model with some nth model going through n-1 models for easier access. Is there anything else to it that I am missing besides the shortcut?

推荐答案

1. 逻辑，好吧，这听起来可能有点弱，但可以说我有一个与我有账户的供应商，我想查看整个此供应商的帐户历史记录"，因此我可以直接从供应商处访问帐户历史记录.

1. Logic, OK it might sound a bit weak for this but it would be logical to say that "I have a supplier who has an account with me, I want to see the entire account history of this supplier", so it makes sense for me to be able to access account history from supplier directly.

效率，这对我来说是我使用 :through 的主要原因，仅仅是因为这会发出一个 join 语句而不是调用供应商，然后帐户，然后是 account_history.注意到数据库调用的次数了吗?

Efficiency, this for me is the main reason I would use :through, simply because this issues a join statement rather than calling supplier, and then account, and then account_history. noticed the number of database calls?

• 使用:through，1 次调用获取供应商，1 次调用获取account_history(rails 自动使用:join 通过帐户检索)

• using :through, 1 call to get the supplier, 1 call to get account_history (rails automatically uses :join to retrieve through account)

使用正常关联，1次调用获取供应商，1次调用获取帐户，1次调用获取account_history

using normal association, 1 call to get supplier, 1 call to get account, and 1 call to get account_history

这就是我的想法 =) 希望它有所帮助！

That's what I think =) hope it helps!

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