Multiple INNER JOINs in a Rails has_many relationship? - ruby-on-rails-3

Question

I have the following models:
class Image < ActiveRecord::Base
belongs_to :gallery
has_many :bookmarks
has_many :gallery_tags, :foreign_key => :gallery_id
end
class Bookmark < ActiveRecord::Base
belongs_to :user
belongs_to :image
has_many :gallery_tags, :through => :image, :source => :gallery_tags
end
class GalleryTag < ActiveRecord::Base
belongs_to :gallery
belongs_to :tag
end
class Gallery < ActiveRecord::Base
belongs_to :provider
has_many :images
belongs_to :user
has_many :gallery_tags
has_many :tags, :through => :gallery_tags
end
class Tag < ActiveRecord::Base
end
class User < ActiveRecord::Base
has_many :bookmarks
has_many :galleries
end
I'd like to be able to do
User.find(1).bookmarked_tags
and retrieve all tags associated with all of the user's bookmarked images via the galleries associated with the images. Tags are associated with galleries.
The query would end up looking like this:
SELECT
tags.*
FROM
tags
INNER JOIN gallery_tags ON gallery_tags.tag_id = tags.id
INNER JOIN images ON gallery_tags.gallery_id = images.gallery_id
INNER JOIN bookmarks ON images.id = bookmarks.image_id
WHERE
bookmarks.user_id = 1
GROUP BY
tags.id;
I've tried adding
has_many :tags, :through => :gallery_tags, :foreign_key => :gallery_id
to Image, which causes Image.find(34).tags to result in
SELECT `tags`.* FROM `tags` INNER JOIN `gallery_tags` ON `tags`.id = `gallery_tags`.tag_id WHERE ((`gallery_tags`.gallery_id = 34))
(it's not using the image's gallery_id in the query), and then I've tried adding
has_many :bookmarked_tags, :through => :bookmarked_images, :source => :tags
to User, which causes User.find(1).bookmarked_tags to result in
ActiveRecord::HasManyThroughSourceAssociationMacroError: Invalid
source reflection macro :has_many :through for has_many
:bookmarked_tags, :through => :bookmarked_images. Use :source to
specify the source reflection.
So: how can I get Rails to run the query I posted with User.find(1).bookmarked_tags?

Answer 1

there are two solutions
First Solution:
Create a view inside your database that acts like a join table:
CREATE VIEW user_bookmarks_tags (
SELECT
bookmarks.user_id AS user_id
gallery_tags.tag_id AS tag_id
FROM
gallery_tags
INNER JOIN images ON gallery_tags.gallery_id = images.gallery_id
INNER JOIN bookmarks ON images.id = bookmarks.image_id
GROUP BY
user_id, tag_id
)
(you can do this inside a migration)
This view acts like a "table" with the columns user_id | tag_id
Now we can adapt our models using has_and_belongs_to_many:
user.rb
class User < ActiveRecord::Base
has_many :bookmarks
has_many :galleries
# we use our magic (view) join table here!
has_and_belongs_to_many :tags, :join_table => :user_bookmarks_tags
end
tag.rb
class Tag < ActiveRecord::Base
# we use our magic (view) join table here
has_and_belongs_to_many :users, :join_table => :user_bookmarks_tags
end
Now you can do: User.find(1).tags or Tag.find(1).users :)
Second Solution
Do the join manually without a view:
define the missing relations (needed for the automated joins foreign_key lookup):
tag.rb
class Tag < ActiveRecord::Base
has_many :gallery_tags
end
gallery_tag.rb
class GalleryTag < ActiveRecord::Base
belongs_to :gallery
belongs_to :tag
# new:
has_many :images, :through => :gallery
end
Now we can add a bookmarked_tags method to our user.rb
class User < ActiveRecordBase
has_many :bookmarks
has_many :galleries
def bookmarked_tags
Tag.joins(:gallery_tags => {:images => :bookmarks}).where('bookmarks.user_id = ?', self.id).group('tags.id')
end
end