限制has_many关系中关联数的最佳实践？ [英] Best practice for limiting the number of associations within a has_many relationship?

问题描述

说我有两个模型-用户和帐户。每个帐户最多可以与n个用户相关联，并且一个用户只能与一个帐户相关联。

Say that I have two models- Users and Accounts. Each account can have at most n users associated with it, and a user can only be associated with one account.

说用户

belongs_to :account

和帐户

has_many :users

但是，我不清楚最佳做法是什么时候通过has_many声明限制关联的数量。我知道有一个：limit参数，但这仅限制了返回的关联数，而不是能够存在的关联数。

However, I'm not clear on the best practice when it comes to limiting the number of associations through that has_many declaration. I know that there is a :limit argument, but that that only limits the number of associations returned, not the number that are able to exist.

我怀疑答案是是使用类似：before_add的东西。但是，该方法似乎仅适用于通过<<< 。因此，当您使用

I suspect that the answer is to use something like :before_add. However, that approach seems only to apply to associations created via << . So it would get called when you used

@account.users << someuser 

但如果您使用

@account.users.create

我也考虑过在用户模型中使用before_save来实现限制可能更实际，但是在用户模型中实现帐户业务规则似乎有点困难。

I had also considered that it might be more practical to implement the limit using before_save within the User model, but it seems like it would be a bit off to implement Account business rules within the User model.

限制关联数的最佳做法是什么？

What is the best practice for limiting the number of associations?

编辑：每个帐户的n个用户将是一些存储在各个帐户中的业务数据，而不是

the n users per account would be some business data that is stored within the individual accounts, rather than being a straight up magic number that would be floating around willy nilly in the code.

推荐答案

首先，如果您的users表中有外部表，密钥account_id，那么您需要使用

At first, if your users table has foreign key account_id then you need to use

class User
  belongs_to :account
end

通过这种方式，您将确保用户只能与一个帐户相关联。

In this way you will ensure that User can be associated just to one Account.

如果您想限制该帐户可以拥有例如最多3个用户，则可以定义以下验证：

If you want to limit that Account can have e.g. at most 3 users then you can define the following validation:

class User
  validates_each :account do |user, attr, value|
    user.errors.add attr, "too much users for account" if user.account.users.size >= 3
  end
end

，因此，如果帐户已有3个用户，则将无法为该帐户创建新用户。

and as a result you will not be able to create new user for account if account has already 3 users.

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