def num_enrolled_in(self, course_id):
        """
        Returns the count of active enrollments in a course.

        'course_id' is the course_id to return enrollments
        """

        enrollment_number = super(CourseEnrollmentManager, self).get_query_set().filter(
            course_id=course_id,
            is_active=1
        ).count()

        return enrollment_number