ActiveRecord 外连接

Question

在 Google 群组中，我添加了我的问题 - here infact！

我的问题是如何用“Rails 方式”来表达这个外连接：

SELECT 
`employees`.`id` AS t0_r0, `employees`.`name` AS t0_r1, `employees`.`last_seen` AS t0_r2, `employees`.`created_at` AS t0_r3, `employees`.`updated_at` AS t0_r4, `employees`.`punch_clock_id` AS t0_r5, `employees`.`account_id` AS t0_r6, `employees`.`born_at` AS t0_r7, `entrances`.`id` AS t1_r0, `entrances`.`employee_id` AS t1_r1, `entrances`.`clocked_at` AS t1_r2, `entrances`.`created_at` AS t1_r3, `entrances`.`updated_at` AS t1_r4, `entrances`.`entrance_type` AS t1_r5 
FROM 
    `employees` 
LEFT OUTER JOIN 
    `entrances` ON `entrances`.`employee_id` = `employees`.`id` AND (`entrances`.`clocked_at` BETWEEN '2015-01-01' AND '2015-01-31’) 
WHERE 
    `employees`.`account_id` = 2 

我的第一反应是：

current_user.account.employees.includes(:entrances).where( 'entrances.clocked_at' => @month_range)

但这确实只为员工提供了入口（除非我松开声明的“位置”部分，在这种情况下我会得到所有员工（以及所有 25 万个入口！！！））

Answer 1

如果您想包含 employees 而没有 entrances，则必须通过调用 entrances.id IS NULL 在您的 where 语句中明确包含该情况。

current_user.account.employees.includes(:entrances).where( 'entrances.id IS NULL OR entrances.clocked_at = ?', @month_range)

根据@month_range 包含的内容，最后一部分可能看起来略有不同。