查找所有未上课的学生（铁路） [英] Find all students not enrolled in a class (rails)

问题描述

曾经试图弄清楚这一点，但我真的很沮丧。

Been trying to figure this one out for a while and I'm really stumped.

所以我有一个舞蹈课模型

So I have a danceclass model

class Danceclass < ActiveRecord::Base
has_many :danceclass_students
has_many :students, through: :danceclass_students

我有一个学生模型：

class Student < ActiveRecord::Base
has_many :danceclass_students
has_many :danceclasses, through: :danceclass_students

我有我的加入模型：

class DanceclassStudent < ActiveRecord::Base
belongs_to :student
belongs_to :danceclass

我使用 @ danceclass.students 来获取所有参加 danceclass 的学生的列表。我如何得到相反的结果？ IE浏览器所有未参加此舞蹈班？

I use @danceclass.students to get a list of all students enrolled in a danceclass. How do I get the opposite of that? IE. All students not enrolled in this danceclass?

推荐答案

的学生然后应用否定where子句删除不想要的子句。

Start with all students then apply a negative where clause to remove the ones you do not want.

Student.joins(:danceclass_students).where.not(danceclass_students: {danceclass_id: @danceclass.id })

加入是必需的，因为 where 语句正试图根据桥表上的ID减少

The joins is needed since the where statement are trying to reduce based on the ID on the bridge table

这将返回所有未参加特定@danceclass的学生，如果它与单个舞蹈课有关。不是您的问题，而是万一其他人遇到这个问题

This will return all students not enrolled in the specific @danceclass if it is related to a single danceclass. Not for your question, but leaving incase anyone else has this problem

Student.where.not(danceclass: @danceclass)

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