在Rails ActiveRecord has_many关联上定制查询

Question

I am using the acts-as-taggable-on gem and want to implement an association on ActsAsTaggableOn::Tag called #related_tags.

The gist of this method is that I want to return all tags that have been tagged in tandem with any specific tag of my choosing. For example, given the following:

Tags
id  name
1   happy
2   bright
3   warm

Posts
id  tags
1   happy,bright
2   bright,warm

Then ActsAsTaggableOn::Tag.find(1).related_tags should include the Tag instances for bright (2), but ActsAsTaggableOn::Tag.find(2).related_tags should include both the instances for happy and warm (1 and 3).

I got the sql working in a sql shell, which is:

SELECT * FROM tags WHERE id IN (SELECT tags.id FROM taggings JOIN tags ON tags.id = taggings.tag_id WHERE taggings.taggable_id IN (SELECT taggable_id FROM taggings WHERE taggings.tag_id = #{tag.id}) AND tags.id <> #{tag.id});

and tried to add it to the association as such:

ActsAsTaggableOn::Tag.class_eval do
   has_many :related_tags, ->(tag) { where( <above sql>, :tag => tag.id) },:class_name => "ActsAsTaggableOn::Tag", :foreign_key => 'id'
end

Which seems like it should work to me... but the SQL that it generates is:

SELECT "tags".* FROM "tags" WHERE "tags"."id" = $1 AND (tags.id IN (SELECT tags.id FROM taggings JOIN tags ON tags.id = taggings.tag_id WHERE taggings.taggable_id IN (SELECT taggings.taggable_id FROM taggings WHERE taggings.tag_id = 10) AND tags.id <> 10))  [["id", 10]]

So the problem here is the extra "tags"."id" = $1 where $1 is the current tag id... but I don't know how that is getting added or how to take it off. If I take the generated sql and remove that criteria, it works fine. Does anyone else know why this is added and how to get rid of it?

Relevant docs are here , but aren't of much help.

Thanks in advance!

Answer 1

I had a problem where I was doing a .where on a has_many through relationship - for example: Update.first.tags.where(...). What I found out is that it was only querying tags already associated with that Update. So the call was first querying Tags currently associated with Update.first, then my SQL query.

Something similar could be happening in your case.

Also, another point of interest you have , tag: tag.id. Those are the only two spots I can think of that would be adding the "tags"."id" = $1