How to Find Courses Not Enrolled by a Specific Student Using a Left Outer Join in Rails 4?

Left Outer Join in Rails 4: Querying for Courses Not Associated with a Student

In Rails 4, you have three models: Student, Course, and StudentEnrollment. You want to query for a list of courses that are not associated with a certain student in the StudentEnrollments table.

Using joins() method, you want to execute the following SQL 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

To achieve this in Rails 4, you can use the joins() method and pass a string that represents the join-SQL. However, it's recommended to use Rails-standard table naming for clarity:

joins("LEFT JOIN student_enrollments ON courses.id = student_enrollments.course_id")

This will perform a left outer join between the Courses and StudentEnrollments tables, and retrieve all courses that are not associated with the specified student.

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