Question

For the purpose of this question, let’s assume I’m building Stack Overflow.

I’ve got a many-to-many relationship via an association table for Posts and Tags, and I’d like to search for posts that contain multiple tags.

I can search for posts that contain a single tag like this:

scope :has_tag, lambda { |tag| includes(:tags).where("lower(tags.name) = ?", tag.downcase) }

Post.has_tag("Python")

For searching for multiple tags, I’ve tried building up a query using a loop:

def self.has_tags(tags)
  query = includes(:tags)
  tags.each do |tag|
    query = query.where("lower(tags.name) = ?", tag)
  end
  return query
end

This results in the query ... WHERE (lower(tags.name) = 'tag1') AND (lower(tags.name) = 'tag2'), which doesn’t match anything.

I’ve also tried using .where('tags.name IN (?)', tags), but I think that matches any post that contains one of the tags instead of posts that contain all the tags.

Now I’m thinking that perhaps I should use the ID of the tags, but that sounds like it could result in multiple queries. Is this inefficient?

... WHERE tag_id IN (SELECT id FROM tag WHERE name IN ('tag1','tag2'))

---

What’s the best way to search for multiple tags?
(I’d prefer to understand the syntax and do it manually rather than using a gem, since I’m new to Rails.)

Answer 1

I ended up using the following:

scope :has_tags, lambda { |tags| includes(:tags)
  .where("lower(tags.name) IN (?)", tags.collect { |tag| tag.downcase } )
  .group('"posts"."id"')
  .having('COUNT(DISTINCT "tags"."id") = ?', tags.count) }