How Can I Use LEFT OUTER JOIN in Rails 4 to Find Courses Not Enrolled by a Specific Student?

LEFT OUTER JOIN with Rails 4

In Rails, you can utilize LEFT OUTER JOIN to retrieve information from multiple tables, even if the data does not exist in all tables.

In your scenario, you wish to retrieve courses not associated with a particular student. The provided SQL query can be implemented using Rails 4's Active Record:

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(active: true)

Note the use of joins to specify the join condition, using the alternative syntax of passing a string. Additionally, the Rails-standard table naming convention has been employed for clarity. This query will fetch courses that are not enrolled by a given student.

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