How to Find Unenrolled Courses in a Rails Application Using ActiveRecord?

Retrieving Courses Not Associated with a Student Enrollment

Question:

Using a Rails application with the following models: Student, Course, and StudentEnrollment, how can I query for a list of courses in the Courses table that are not associated with a student via the StudentEnrollment table?

Solution:

To achieve this, you can utilize the JOIN operation in ActiveRecord. However, unlike the joins() method, which accepts a table name as an argument, you can also pass a raw SQL string to specify custom join conditions.

Specifically, for the given query:

SELECT *
FROM Courses c LEFT JOIN StudentEnrollment se ON c.id = se.course_id
WHERE se.id IS NULL AND se.student_id = <SOME_STUDENT_ID_VALUE> and c.active = true

In Rails, you can execute this query as follows:

Course.joins("LEFT JOIN student_enrollments ON courses.id = student_enrollments.course_id")
      .where("student_enrollments.id IS NULL AND student_enrollments.student_id = ?, courses.active = ?", <SOME_STUDENT_ID_VALUE>, true)

This query will retrieve all courses that are not associated with the specified student in the student_enrollments table, while also ensuring that only active courses are included.

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