如何獲得使用活動記錄的朋友的朋友？

Question

我已經通過使用與http://railscasts.com/episodes/163-self-referential-association中相同的程序實現了朋友關係。我想從同一個城市找朋友的朋友，但我無法弄清楚如何得到朋友的朋友和來自同一個城市的朋友。我對Ruby on Rails和主動記錄頗爲陌生，對我來說很裸露。

我打過電話：

User.find_by(first_name: user_name).friendships.friendships(:f_of_f).cities.where(name: "london").pluck(:f) 

，並沒有工作。

我的代碼如下：

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 

和城市：

class City < ActiveRecord::Base 
    has_many :user_cities 
    has_many :users, through: :user_cities 
end 

和user_city：

class UserCity < ActiveRecord::Base 
    belongs_to :city 
    belongs_to :user 
end 

和友誼：

class Friendship < ActiveRecord::Base 
    belongs_to :user 
    belongs_to :friend, class_name: 'User' 
end 

錯誤我試圖解決方案：

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的是不是Neo4j的寶石，而你正在運行到的重要原因，其中的ActiveRecord是困難的。

我想，這大概是最有效的，你會得到，而無需編寫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)) 

或者你可以做這樣的事情：

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)