ACTIVE 记录查询以从三个关联模型中获取数据答案

【问题标题】：ACTIVE Record Query to Fetch data from three associated modelsACTIVE 记录查询以从三个关联模型中获取数据
【发布时间】：2017-11-20 02:44:43
【问题描述】：

我是编程和 Rails 的新手。在我的 Rails 应用程序中，我有称为 users、listings、reservations 的模型。一个用户有_many 个列表和预订。一个列表有_many 个预订和预订属于一个列表和用户。

    class User < ActiveRecord::Base
     has_many :listings
     has_many :reservations
    end

    class Listing < ActiveRecord::Base
    belongs_to :user
    has_many :reservations
    end


    class Reservation < ActiveRecord::Base
    belongs_to :user
    belongs_to :listing
    end

我想要一个查询来获取所有包含以下详细信息的预订

 RESERVATIONS # | LISTING OWNER NAME  | LISTING OWNER EMAIL | LISTING OWNER PHONE | GUEST NAME | GUEST EMAIL | CLASS OF LISTING | YEAR OF LISTING | NIGHTLY RENTAL RATE OF LISTING| TOTAL RENTAL AMOUNT

架构：

create_table "users", force: :cascade do |t|
    t.string   "email",
    t.string   "fullname",   
    t.string   "phone_number"
    end  

 create_table "listings", force: :cascade do |t|
    t.string   "vehicle_type",   
    t.string   "listing_name" ,            
    t.string   "make",                         
    t.string   "model",                        
    t.string   "year", 
    t.integer  "daily_rent",
    t.integer  "user_id"   
end

create_table "reservations", force: :cascade do |t|
    t.integer  "user_id",  
     t.integer  "total",
     t.integer  "listing_id"
 end

作为房源所有者和访客，两者都是用户我可以通过一个简单的 SQL 查询来获取所有预订详细信息，包括客人的电子邮件和电话以及相应预订的详细信息

  select r.id , u.email, u.fullname ,l.listing_name,l.year,l.daily_rent,r.total  from reservations r , users u ,listings l where u.id = r.user_id and l.id = r.listing_id;

1)我无法提取列表所有者名称 |所有者电子邮件 | OWNER PHONE，但我能够获取客人姓名 |来宾电子邮件 |来宾电话。如何获取 LISTING OWNER 详细信息？？

2) 如何在 Rails Active Record 中编写上述 SQL 查询，以便在模型或控制器中使用它来获取结果？

非常感谢任何帮助。在此先感谢！

【问题讨论】：

标签： mysql sql ruby-on-rails ruby rails-activerecord

【解决方案1】：

您可以定义自定义外键以及名称，以便以更简洁的方式检索记录：

class User < ActiveRecord::Base
  has_many :listings, foreign_key: :owner_id
  has_many :reservations, foreign_key: :guest_id
end

class Listing < ActiveRecord::Base
  belongs_to :owner, class_name: 'User'
  has_many   :reservations
end

class Reservation < ActiveRecord::Base
  belongs_to :guest, class_name: 'User'
  belongs_to :listing
end

通过重新定义模型，现在您可以像这样轻松地查询它们：

reservation = Reservation.find_by(id: 1)
owner = reservation.listing.owner
guest = reservation.guest
total = reservation.total

# 'owner' and 'guest' are User instances. So you can invoke any fields on them that are defined in the model
owner.email
owner.fullname
owner.phone_number

顺便说一句，要支持上述定义，您必须运行以下迁移：

class AddReferencesInListingsAndReservations < ActiveRecord::Migration
  def change
    remove_column :listings,     :user_id,    :integer
    remove_column :reservations, :user_id,    :integer
    remove_column :reservations, :listing_id, :integer

    add_reference :listings,     :owner, foreign_key: { to_table: :users }
    add_reference :reservations, :guest, foreign_key: { to_table: :users }
    add_reference :reservations, :listing
  end
end

这将导致以下架构：

create_table "listings", force: :cascade do |t|
  t.string   "vehicle_type",   
  t.string   "listing_name" ,            
  t.string   "make",                         
  t.string   "model",                        
  t.string   "year", 
  t.integer  "daily_rent",
  t.integer  "owner_id"
  t.index    "owner_id",   name: "index_listings_on_owner_id"
end

create_table "reservations", force: :cascade do |t|
  t.integer  "guest_id",  
  t.integer  "total",
  t.integer  "listing_id"
  t.index    "guest_id",   name: "index_listings_on_guest_id"
  t.index    "listing_id", name: "index_listings_on_listing_id"
end

add_foreign_key "listings",     "users", column: "owner_id"
add_foreign_key "reservations", "users", column: "guest_id"
add_foreign_key "reservations", "listings"

【讨论】：

【解决方案2】：

当您说“列表所有者”时，我假设您的意思是用户关系。

reservation = Reservation.find(id) # Retrieves the reservation
reservation.user # Retrieves the associated user object
reservation.user.fullname # Listing owner name...

【讨论】：