How to find all that has multiple relationships in many to many with Rails3 in a elegant way? - ruby-on-rails-3

Question

I have a User class with
class User < ActiveRecord::Base
has_many :forum_subscriptions
has_many :forums, :through => :forum_subscriptions
And a Forum class with
class Forum < ActiveRecord::Base
has_many :users
I want to find all the users that are subscribed to forum "sport", forum "TV" and forum "Hobby"
What is the most elegant way to do it?
(I have a lot of ugly stuff in my mind :-)

Answer 1

Here is something I would use:
User.joins(:forums).where("forums.title IN (?)", %w(sport TV Hobby)).group("users.id")
I assumed the column for the forum's title is 'title'. Change it when you're using some other name. You can put this inside a nice scope inside the User Model to make it a bit more dynamic.
scope :subscribed_to, lambda { |forum_titles| joins(:forums).where("forums.title IN (?)", forum_titles).group("users.id") }
Now you can call this scope from inside the Controller or some other place:
User.subscribed_to(%w(sport TV Hobby)) # => Get all users that are subscribed to sport, TV and Hobby
User.subscribed_to(["sport", "TV", "Hobby"]) # => Does the same
User.subscribed_to(%w(Hobby)) # => Get all users that are subscribed to Hobby
User.subscribed_to("Hobby") # => Does the same
I assumed you have the following relationships:
class User < ActiveRecord::Base
has_many :forum_subscriptions
has_many :forums, :through => :forum_subscriptions
end
class Forum < ActiveRecord::Base
has_many :forum_subscriptions
has_many :users, :through => :forum_subscriptions
end
class ForumSubscription < ActiveRecord::Base
belongs_to :user
belongs_to :forum
end
Hope this is what you needed. :)