轨道3：对象使用的has_many链接：通过协会 [英] Rails 3: Object chaining with has_many :through associations

问题描述

我想获得如何在Rails的协会工作一抓，具体地，当，当不写明确的SQL code。

I'm trying to get a grasp of how to work with associations in Rails, specifically, when and when not to write explicit SQL code.

在我的申请，我有四个模型，定义如下：

In my application, I have four models, which are defined as follows:

class User < ActiveRecord::Base
    has_many :comments
    has_many :geographies
    has_many :communities, through: :geographies

class Comment < ActiveRecord::Base
    belongs_to :user

class Community < ActiveRecord::Base
    has_many :geographies
    has_many :users

class Geography < ActiveRecord::Base
    belongs_to :user
    belongs_to :community

用户可以发表评论，并通过地理表关联到一个或多个社区（地理表存储 USER_ID 和 community_id ）。

Users can post comments, and are associated to one or more communities through the geography table (the geography table stores user_id and community_id).

我有一个索引行动列出所有的意见，我想由社区进行过滤。给定一个评论的对象，我可以通过 comment.user 用户对象，但我不能链条超出了（也就是说，像评论。 user.geography（0）.community 不工作）。

I have an index action listing all comments, and I would like to filter by community. Given a comment object, I can get the user object via comment.user, but I can't chain beyond that (i.e., something like comment.user.geography(0).community doesn't work).

看来这个对象链接是轨道的一个重要功能，但它的has_many工作：通过协会？鉴于我的例子，它是通过使用对象链接摆脱comment对象的社会目标，否则我就需要编写SQL来获得用户以外的任何给出的评论对象时？

It seems this object chaining is a key feature of rails, but does it work with has_many :through associations? Given my example, is it possible to get the community object from the comment object by using object chaining, or would I need to write the SQL to get anything other than the user when given the comment object?

推荐答案

由于用户与多个社区相关联的，你需要告诉你想要的社区ActiveRecord的（或原始SQL）：

Since User is associated with multiple communities, you will need to tell ActiveRecord (or raw SQL) which community you want:

comment.user.communities #=> should give you all the communities

如果你不特别在意获取所有社区和只想得到任何社区

If you don't particularly care for getting all communities and just want to get any community

comment.user.communities.first #=> should give you the first community

不过，一般你会有兴趣在一个特定的社区，基于一个条件。

But, generally you will be interested in one particular community, based on a condition.

comment.user.communities.where(name: 'Europe') #=> should give you the European community.

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