Rails 4左外部联接问题返回错误记录

Question

I have 2 tables: appointments and doors. Door has one appointment and appointment belongs to a door. The appointments table have the door_id.

The query I wrote is:

scope :not_in_appointments, -> { joins("left outer join appointments on appointments.door_id = doors.id") }

ISSUE: The scope is still returning the doors that are in the appointments table.

GOAL: We want to get doors that are NOT in the appointments table.

The left outer join is suppose to do that. Can someone please let us know why this query is not working?

Thanks.

Answer 1

效果也一样，在范围内添加了一个额外的where子句：

scope :not_in_appointments, -> { joins("left outer join appointments on appointments.door_id = doors.id").where("appointments.id IS NULL") }

Answer 2

Let try this one

scope :not_in_appointments, -> { where("id NOT IN (SELECT door_id FROM appointments)") }

The performance is much better than using 'left outer join' because we are reducing number of redundant records as soon as possible.