Rails 中的嵌套连接答案

【问题标题】：Nested Joins in RailsRails 中的嵌套连接
【发布时间】：2018-02-06 08:46:14
【问题描述】：

我的 Rails 应用程序中有 3 个模型 Company, User, Loan

company.rb

class Company < ApplicationRecord
  has_many :users
end

Company id: 1, name: "A", created_at: "2018-02-06 07:34:17", updated_at: "2018-02-06 07:34:17"
Company id: 2, name: "B", created_at: "2018-02-06 07:34:19", updated_at: "2018-02-06 07:34:19"
Company id: 3, name: "C", created_at: "2018-02-06 07:34:21", updated_at: "2018-02-06 07:34:21"

user.rb

class User < ApplicationRecord
  belongs_to :company
  has_many :loans
end

User id: 1, name: "Sachin", company_id: 1, created_at: "2018-02-06 07:35:53", updated_at: "2018-02-06 08:02:41", user_status: true
User id: 2, name: "Ghanshyam", company_id: 1, created_at: "2018-02-06 07:36:01", updated_at: "2018-02-06 08:02:41", user_status: false
User id: 3, name: "Anand", company_id: 1, created_at: "2018-02-06 07:36:06", updated_at: "2018-02-06 08:02:41", user_status: false
User id: 4, name: "Ghanshyam Rahul", company_id: 1, created_at: "2018-02-06 07:36:15", updated_at: "2018-02-06 08:02:41", user_status: false
User id: 5, name: "Anand", company_id: 3, created_at: "2018-02-06 07:43:56", updated_at: "2018-02-06 08:02:41", user_status: false
User id: 6, name: "Ghanshyam", company_id: 3, created_at: "2018-02-06 07:43:58", updated_at: "2018-02-06 08:02:41", user_status: false

loan.rb

class Loan < ApplicationRecord
  belongs_to :user
end

Loan id: 1, name: "loan 1", user_id: 1, created_at: "2018-02-06 07:44:46", updated_at: "2018-02-06 07:44:46"
Loan id: 5, name: "loan 5", user_id: 1, created_at: "2018-02-06 07:44:54", updated_at: "2018-02-06 07:44:54"
Loan id: 6, name: "loan 6", user_id: 6, created_at: "2018-02-06 07:45:33", updated_at: "2018-02-06 07:45:33"
Loan id: 7, name: "loan 7", user_id: 6, created_at: "2018-02-06 07:45:34", updated_at: "2018-02-06 07:45:34"
Loan id: 8, name: "loan 8", user_id: 6, created_at: "2018-02-06 07:45:36", updated_at: "2018-02-06 07:45:36"

如果有用户有任何贷款，我想写下获取公司列表的连接查询（单个查询），如下所示：

Company id: 1, name: "A", created_at: "2018-02-06 07:34:17", updated_at: "2018-02-06 07:34:17"
Company id: 3, name: "C", created_at: "2018-02-06 07:34:21", updated_at: "2018-02-06 07:34:21"

【问题讨论】：

标签： mysql ruby-on-rails database join

【解决方案1】：

Company.joins(users: [:loans]).group('companies.id')

为避免可能的公司重复，我将按公司 ID 字段对公司表进行分组。

【讨论】：

【解决方案2】：

Company.joins(users: :loans).distinct

会给你所有有贷款用户的公司

您创建join 查询，该查询将与他们的用户一起加入公司，并为您提供与用户一起的公司，然后将用户与他们的贷款一起加入。由于它使用的是内部连接，当用户没有公司时，他们不会出现，他们的公司也不会出现

【讨论】：