ruby-on-rails - rails : Find rows without connection in HABTM relation

Question

我有两个模型，Users 和 Leads 与 HABTM 关系相关:

class Lead < ActiveRecord::Base
  has_and_belongs_to_many :users
end

class User < ActiveRecord::Base
  has_and_belongs_to_many :leads
end

我现在如何才能只获得那些与用户无关的线索？

提前致谢!

Answer 1

您正在寻找的是 anti join .

有三种标准方法可以实现这一点，

1. 使用空左外连接
2. 使用带有 NOT 和 IN 关键字的子查询的 where 子句
3. 使用带有 NOT 和 EXISTS 关键字的 where 子句

基本上，EXISTS 关键字将检查子查询是否返回任何行并将其报告为匹配项，NOT 显然会否定真正的匹配项。

这是我的首选方式(使用NOT & EXISTS)

class User < ActiveRecord::Base
  has_and_belongs_to_many :leads
  def self.without_leads
    where(<<-SQL)
      NOT EXISTS (SELECT 1 
        FROM   leads_users 
        WHERE  users.id = leads_users.user_id) 
    SQL
  end
end

class Lead < ActiveRecord::Base
  has_and_belongs_to_many :users
  def self.without_users
    where(<<-SQL)
      NOT EXISTS (SELECT 1 
        FROM   leads_users 
        WHERE  leads.id = leads_users.lead_id) 
    SQL
  end

 def self.not_connected_to(user)
    where(<<-SQL, user.id)
      NOT EXISTS (SELECT 1 
        FROM   leads_users 
        WHERE  leads.id = leads_users.lead_id
        AND leads_users.user_id = ?
        ) 
    SQL
  end
end

这是一个使用 arel 的非 SQL 方法

class User < ActiveRecord::Base
  has_and_belongs_to_many :leads
  def self.without_leads
    habtm_table = Arel::Table.new(:leads_users)
    join_table_with_condition = habtm_table.project(habtm_table[:user_id])
    where(User.arel_table[:id].not_in(join_table_with_condition))
  end
end

class Lead < ActiveRecord::Base
  has_and_belongs_to_many :users
  def self.without_users
    habtm_table = Arel::Table.new(:leads_users)
    join_table_with_condition = habtm_table.project(habtm_table[:lead_id])
    where(Lead.arel_table[:id].not_in(join_table_with_condition))
  end
end

here is an example repo

寻找没有线索的用户

User.where(user_id: 1).without_leads

寻找没有用户的线索

Lead.without_users

查找未连接到特定用户的线索

Lead.not_connected_to(user)

链接排序

Lead.without_users.order(:created_at)