howto get friends of friends using active record? - ruby-on-rails

Question

I have implemented friends relationsships by using the same procedure as in http://railscasts.com/episodes/163-self-referential-association. I want to fetch friends of friends from the same city, but I cannot figure out howto get friends of friends and from the same city.
I tried calling:
User.find_by(first_name: user_name).friendships.friendships(:f_of_f).cities.where(name: "london").pluck(:f)
and it didnt work.
my code is the following:
class User < ActiveRecord::Base
has_many :friendships
has_many :friends, through: :friendships
has_many :inverse_friendships, class_name: 'Friendship', foreign_key: 'friend_id'
has_many :inverse_friends, through: :inverse_friendships, source: :user
has_many :cities, through: :user_cities
has_many :user_cities
belongs_to :company
has_many :comments, dependent: :delete_all
has_many :reviews, dependent: :delete_all
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :trackable, :validatable
def self.friends_reviews_on_company(user_id, city_id)
User.find_by(first_name: user_name).friendships.friendships(:f_of_f).cities.where(name: "london").pluck(:f)
end
end
and city:
class City < ActiveRecord::Base
has_many :user_cities
has_many :users, through: :user_cities
end
and user_city:
class UserCity < ActiveRecord::Base
belongs_to :city
belongs_to :user
end
and friendship:
class Friendship < ActiveRecord::Base
belongs_to :user
belongs_to :friend, class_name: 'User'
end
Error I tried solution:
PG::UndefinedTable: ERROR: missing FROM-clause entry for table "friends_friends_of_friends_join"
LINE 1: ...ndships" "friendships_friends_of_friends_join" ON "friends_f...
^
: SELECT "users".* FROM "users" INNER JOIN "user_cities" ON "user_cities"."user_id" = "users"."id" INNER JOIN "cities" ON "cities"."id" = "user_cities"."city_id" INNER JOIN "friendships" ON "users"."id" = "friendships"."friend_id" INNER JOIN "friendships" "friendships_friends_of_friends_join" ON "friends_friends_of_friends_join"."id" = "friendships_friends_of_friends_join"."friend_id" WHERE "friendships"."user_id" = $1 AND "friendships_friends_of_friends_join"."user_id" = $2 AND "cities"."name" = 'London' LIMIT 1 OFFSET 0
Rendered search/index.html.erb within layouts/application (8.4ms)
Completed 500 Internal Server Error in 27ms
ActionView::Template::Error (PG::UndefinedTable: ERROR: missing FROM-clause entry for table "friends_friends_of_friends_join"
LINE 1: ...ndships" "friendships_friends_of_friends_join" ON "friends_f...
^
: SELECT "users".* FROM "users" INNER JOIN "user_cities" ON "user_cities"."user_id" = "users"."id" INNER JOIN "cities" ON "cities"."id" = "user_cities"."city_id" INNER JOIN "friendships" ON "users"."id" = "friendships"."friend_id" INNER JOIN "friendships" "friendships_friends_of_friends_join" ON "friends_friends_of_friends_join"."id" = "friendships_friends_of_friends_join"."friend_id" WHERE "friendships"."user_id" = $1 AND "friendships_friends_of_friends_join"."user_id" = $2 AND "cities"."name" = 'London' LIMIT 1 OFFSET 0):

Answer 1

ActiveRecord isn't the neo4j gem, and you're running into one of the big reasons where ActiveRecord is harder.
I think this is about the most efficient you'll get without writing SQL:
# Getting a query for multiple friends, even if we're only expecting one so that we can use `includes` method
friend_user_ids = User.where(first_name: user_name).includes(:friendships).map(&:friendships).flatten.map(&:friend_id)
fof_user_ids = Friendship.find(user_id: friend_user_ids).pluck(:friend_id)
city = City.find(name: 'london')
result = User.find(UserCity.where(user_id: fof_user_ids).where(city_id: city.id).pluck(:user_id))
Alternatively you could do something like this:
city = City.find(name: 'london')
result_ids = User.where(first_name: user_name)
.joins('LEFT JOIN friendships ON users.id=friendships.user_id LEFT JOIN friendships AS friendships2 ON friendships.friend_id=friendships2.user_id LEFT JOIN users_cities ON friendships2.friend_id=users_cities.user_id')
.where('users_cities.city_id = ?', city.id)
.pluck('friendships2.friend_id')
result = User.find(result_ids)