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.