Rails or SQL to get through a few model associations
Timetable
    has_many :enrollments

Enrollment
    belongs_to :timetable
    belongs_to :stream
    has_one :course, :through => stream
    #Stream & Timetable together are unique for an Enrollment

Stream
    has_many :enrollments
    belongs_to :course

Course
    has_many :streams

Timetable.courses

Timetable
    has_many :courses, :through => :enrollments, :source => :course

SELECT c.*
FROM courses c
  LEFT JOIN streams s ON s.course_id = c.id
  LEFT JOIN enrollments e ON e.stream_id = s.id
  LEFT JOIN timetables t ON e.timetable_id = t.id
WHERE t.id = 5

Course.joins(:streams => {:enrollments => :timetables}).to_sql

Course.joins(:streams => {:enrollments => :timetables}).where(some condition on timetables)