Question

Maybe the title is confusing, but I didn’t know how to explain my doubt.

Say I have the following class methods that will be helpful in order to do chainings to query a model called Player. A Player belongs_to a User, but if I want to fetch Players from a particular village or city, I have to fetch the User model.

def self.by_village(village)
  joins(:user).where(:village => "village")
end

def self.by_city(city)
  joins(:user).where(:city => "city")
end

Let’s say I want to fetch a Player by village but also by city, so I would do…

Player.by_city(city).by_village(village).

This would be doing a join of the User twice, and I don’t think that is correct.. Right?

So my question is: What would be the correct way of doing so?

Answer 1

I haven’t tried that, but I would judge the answer to your question by the actual sql query ActiveRecord generates. If it does only one join, I would use it as you did, if this results in two joins you could create a method by_village_and_city.

OK. Tried it now:

1.9.2p290 :022 > Player.by_city("Berlin").by_village("Kreuzberg")
  Player Load (0.3ms)  SELECT "players".* FROM "players" INNER JOIN "users" ON "users"."id" = "players"."user_id" WHERE "users"."city" = 'Berlin' AND "users"."village" = 'Kreuzberg'
 => [#<Player id: 1, user_id: 1, created_at: "2012-07-28 17:05:35", updated_at: "2012-07-28 17:05:35">, #<Player id: 2, user_id: 2, created_at: "2012-07-28 17:08:14", updated_at: "2012-07-28 17:08:14">] 

So, ActiveRecors combines the two queries, does the right thing and I would use it, except:
I had to change your implementation though:

class Player < ActiveRecord::Base
  belongs_to :user
  def self.by_village(village)
    joins(:user).where('users.village' => village)
  end

  def self.by_city(city)
    joins(:user).where('users.city' => city)
  end
end

and what you’re doing is usually handled with parameterized scopes:

class Player < ActiveRecord::Base
  belongs_to :user

  scope :by_village, lambda { |village| joins(:user).where('users.village = ?', village) }
  scope :by_city, lambda { |city| joins(:user).where('users.city = ?', city) }

end