Arel Multi-join example.rb

# The problem: In your Rails 3 project, you have a model Request that models a
# user's request for content related to a specific problem or topic. The user 
# can tag her request with any number of words. These are modeled by a Tag class
# backed by a tags DB table. Since you want unique records per tag, you have a 
# Tagging class backed by a taggings table. Taggings is a a many-to-many table
# with some additional information. Also, other models in the application 
# besides requests can be tagged: The Tagging class defines a polymorphic 
# relationship "taggable" with those models.
#
# So pretty soon you want to look up all the requests that are tagged with
# certain words, say "bunnies" and "bacon".
#
# You can do that with raw SQL:

Request.find_by_sql("
  SELECT * FROM requests
  INNER JOIN taggings ON taggings.taggable_id = requests.id AND taggings.taggable_type = 'Request'
  INNER JOIN tags ON taggings.tag_id = tags.id
  WHERE tags.contents = 'bunnies' OR tags.contents = 'bacon'
")

# But you are using Rails 3. You have Arel. So how do you do the chained inner
# joins from requests to taggings and then taggings to tags? 

# With Arel
Request.joins(:taggings).
        where(:taggings => {
          :id => Tagging.joins(:tag).
                         where(:tags => {:contents  => ["bunnies","bacon"]})
        })

# You probably do not want duplicate Request records
Request.joins(:taggings).
        where(:taggings => {
          :id => Tagging.joins(:tag).
                         where(:tags => {:contents  => ["bunnies","bacon"]})
        }).uniq