Ruby ActiveRecord multiple joins through associations - sql

Question

I'd like to convert
SELECT `users`.* FROM `users`
INNER JOIN `memberships`
ON `memberships`.`user_id` = `users`.`id`
INNER JOIN `roles`
ON `roles`.`id` = `memberships`.`role_id`
WHERE `memberships`.`group_id` = 'NUCC' AND (expiration > '2012-07-02')
ORDER BY `roles`.`rank` DESC
Into an ActiveRecord association.
Groups have many members (class User) through memberships. Each membership has a role (role_id) which maps to another table (roles) and subsequently an AR model (Role). Each role has a rank (integer) associated with it.
I'd like to simply sort the members of a group by the memberships-roles-rank.

Answer 1

Untested, probably has typos, but...
class User < ActiveRecord::Base
has_many :memberships
has_many :roles, :through => :memberships, :uniq => true
end
class Membership < ActiveRecord::Base
belongs_to :user
belongs_to :role
end
class Role < ActiveRecord::Base
has_many :memberships
has_many :users, :through => :memberships, :uniq => true
end
And then, to sort the users by roles.rank:
q = User.joins(:memberships => :users)
q = q.where(:memberships.group_id => 'NUCC')
q = q.where("expressionn > '2012-07-02'")
q = q.order("roles.rank DESC")
puts q.to_sql
AREL lets you join things up like that pretty easily. For instance, you can keep that going with even further INNER JOINS with syntax similar to:
User.joins(:memberships => { :users => :someothermodel })
Just remember to replicate that structure whenever you need to reference something through the JOIN, or just write your own SQL fragment..