How to Perform a LEFT OUTER JOIN in Rails 4 to Find Courses Unenrolled by a Specific Student?

LEFT OUTER JOIN in Rails 4

In Rails 4, it's possible to execute complex SQL queries like a LEFT OUTER JOIN using ActiveRecord's joins method. This method allows you to specify the join conditions as a string.

Consider a scenario with three models: Student, Course, and StudentEnrollment. Suppose you want to retrieve a list of courses that are not enrolled by a specific student.

To achieve this using a LEFT OUTER JOIN, the following SQL query can be employed:

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, this query can be executed using the following code:

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

By utilizing the joins method with a string argument, you can specify the join conditions in detail. This approach provides a flexible way to execute complex database queries in Rails.

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