Question

在ActiveRecord / Ruby on Rails中，如果我有这样的多对多关系设置：

class Blog < ActiveRecord::Base
  has_many :tagged_blogposts;
  has_many :tags, through: :tagged_blogposts;
end

class Tag < ActiveRecord::Base
  has_many :tagged_blogposts
  has_many :blogs, through: :tagged_blogposts
end

class TaggedBlogpost < ActiveRecord::Base
  belongs_to :blog
  belongs_to :tag
end

如何选择具有给定tag_id的所有博客？

到目前为止，我有： TaggedBlogposts.all.where（“tag_id =？”，“1”），返回：

+----+---------+--------+-------------------------+-------------------------+
| id | blog_id | tag_id | created_at              | updated_at              |
+----+---------+--------+-------------------------+-------------------------+
| 2  | 1       | 1      | 2015-08-24 03:05:15 UTC | 2015-08-24 03:05:15 UTC |
| 4  | 2       | 1      | 2015-08-24 03:05:15 UTC | 2015-08-24 03:05:15 UTC |
| 7  | 3       | 1      | 2015-08-24 03:05:15 UTC | 2015-08-24 03:05:15 UTC |
| 15 | 6       | 1      | 2015-08-24 03:05:15 UTC | 2015-08-24 03:05:15 UTC |
| 17 | 7       | 1      | 2015-08-24 03:05:15 UTC | 2015-08-24 03:05:15 UTC |
| 24 | 9       | 1      | 2015-08-24 03:05:15 UTC | 2015-08-24 03:05:15 UTC |
| 33 | 13      | 1      | 2015-08-24 03:05:15 UTC | 2015-08-24 03:05:15 UTC |
| 36 | 15      | 1      | 2015-08-24 03:05:15 UTC | 2015-08-24 03:05:15 UTC |
| 46 | 18      | 1      | 2015-08-24 03:05:15 UTC | 2015-08-24 03:05:15 UTC |
| 47 | 19      | 1      | 2015-08-24 03:05:15 UTC | 2015-08-24 03:05:15 UTC |
+----+---------+--------+-------------------------+-------------------------+

但是从这个连接表中，我想返回相应的博客对象。类似的东西：

Blog.all.where(tag_id:1, through: TaggedBlogposts)

编辑：想出来：

@blogs = Blog.joins(:tags).where("tags.id=?", "#{params[:tagid]}")

Answer 1

这可以为您提供预期的博客：

Blog.joins('tagged_blogposts').where('tagged_blogposts.tag_id = ?', tag_id)

Answer 2

尝试以下代码

Blog.inlcudes(:tagged_blogposts).where('tagged_blogposts.tag_id = ?', 1).references(:tagged_blogposts)

Answer 3

首先查找标签记录：
tag = Tag.find tag_id
然后你可以使用以下方法获取此标签的所有博客：
tag.blogs

从多个到多个连接表访问对象 - RAILS

3 个答案: