Question

noob question here. I have a has_many :through relationship between items and tags. Finding all items associated with a given tag is simple enough:

things=Tag.find(1).items

But what if I want to find all items associated with more than one given tag? I was thinking something like:

things=Tag.find(1).items
Tag.find(2).things # wrong, but you get the idea

Answer 1

You can use the array union operator.

things = Tag.find(1).items | Tag.find(2).items

That will create an object for every item for both tags, which could be way too much depending on what you’re trying to do. If you want something a little more scalable, you can do the lookup on the join table.

things = ItemTags.find_by_sql("
  SELECT item_id, COUNT(tag_id) AS tag_count 
  FROM item_tags
  WHERE tag_id IN (1, 2)
  GROUP_BY item_id
  HAVING tag_count = 2;
").map(&:item)

Just wrote that in browser, so it could be completely wrong. Also, there is probably a way to do that with activerecord finders that would be nicer that a find_by_sql.