Rails：从has-many关联中获取所有关联模型(Rails: Get all associated models from has-many association)

 我需要从其他关联模型中获取所有关联模型，我希望首先运行查询。 
 
 例如，我有Post模型和Tag模型。 我需要获得与一些标签相关的所有帖子。 没有问题，如果我只有一个标签 - 只需要调用'tag.posts'，但如果我有更多，那么一个标签 - 例如，我需要做一些事情，如： 
 
Post.where(id: PostTag.where(tag_id: some_ids).pluck(:category_id).uniq)
 
 我相信Rails有一个内置的解决方案。 那么，谁知道呢？ 

I need to get all associated models from the other associated model, on which I want to run query first.
 
For example, I got Post model and Tag model. I need to get all Posts, which associated with some Tags. There's no problem, if I have only one Tag – just call 'tag.posts', but if I have more, then one Tag – for example, I need to do somethink like:
 
Post.where(id: PostTag.where(tag_id: some_ids).pluck(:category_id).uniq)
 
I belive that Rails have a built-in solution. So, anybody knows it?

原文：https://stackoverflow.com/questions/16551235