Rails ActiveRecord通过命名范围中的标签查找问题

Question

I want the equivalent of SO search by tag, so I need an exists query but I also still need to left join on all tags. I've tried a couple of approaches and I'm out of ideas.

The Qustion - Tag relationship is through has_and_belongs_to_many both ways (ie I have a QuestionTags joiner table)

eg

Question.join(:tags).where('tag.name = ?', tag_name).includes(:tags)

I would expect this to do what I need but actually it just mashes up the includes with the join and I just end up with basically an inner join.

Question.includes(:tags)
   .where("exists (
               select 1 from questions_tags 
                where question_id = questions.id 
                  and tag_id = (select id 
                                  from tags 
                                 where tags.name = ?))", tag_name) 

This fetches the correct results but a) is really ugly and b) gives a deprecation warning as again it seems to confuse the includes with the join :

DEPRECATION WARNING: It looks like you are eager loading table(s) (one of: questions, tags) that are referenced in a string SQL sn ippet. For example:

 Post.includes(:comments).where("comments.title = 'foo'") 

Note I'm trying to write these as named scopes.

Let me know if the question isn't clear. Thanks in advance.

Answer 1

OK, got it. I know no built in syntax to do it. I have used an alternative before, You can do like this:

Question.include(:tags).where("questions.id IN (
#{ Question.joins(:tags).where('tags.name = ?', tag_name).select('questions.id').to_sql})")

You can also join this subquery to your questions table instead of using IN. Alternatively if You are not against adding gems and You are using Postgres, use this gem . It provides really neat syntax for advanced queries.

Answer 2

使用preload而不是includes ：

Question.preload(:tags).where("exists ....