Question

I have a Forum and ForumTag HABTM relationship. I also have an array of variables named @tags . This array contains the names of some ForumTags. I want to be able to query and find all forums that have ALL the values of the array. I currently have:

@forums = Forum.joines(:forum_tags).where(:forum_tags => {:name => @tags}).includes(:forum_tags).all

However, this returns all the forums that have AT LEAST ONE value in the array.

Answer 1

The following will require the forums to have all the forum tags in the @tags array. I am making the assumption that a forum will not have the same forum_tag more than once.

@forums = Forum.joins(:forum_tags).where(:forum_tags => {:name => @tags}).group("forums.id").having(['COUNT(*) = ?', @tags.length]).includes(:forum_tags).all

This will produce an SQL query like the following:

@tags = ['foo', 'bar']

SELECT forums.id, forum_tags.id FROM forums
  LEFT OUTER JOIN forum_tags_forums on forum_tags_forums.forum_id = forums.id
  LEFT OUTER JOIN forum_tags ON forum_tags.id = forum_tags_forums.forum_tag_id
  WHERE forum_tags.name IN ('foo', 'bar')
  GROUP BY forums.id
  HAVING COUNT(*) = 2;

This will group all the rows in the join table by forums that match the given tags. If the COUNT function has the value of the total number of tags that you’re looking for (and there are no duplicate forum/forum_tag pairs) then the forum must contain all the tags.

To get the leftover tags (question asked in the comments):

forum_tags = ForumTag.where(:name => @tags)

@forums_with_leftovers = Forum.select("forums.*, GROUP_CONCAT(forum_tags.name) AS leftover_tags").joins(:forum_tags).where(['forums.id IN (?) AND NOT forum_tags.id IN (?)', @forums, forum_tags]).group("forums.id").all

Each Forum object in @forums_with_leftovers will have an extra attribute leftover_tags that contains a comma separated list of tags in each forum object that is not in the original @tags variable.