gistfile1.rb

  # this is a bit of a hack to make a join on two keys:
  # classroom_id and student_id
  # we pretend classroom_id is the primary key
  # then we add a condition for student_id
  # the result is an association joined on two keys
  has_many :enrollments,
    foreign_key: :classroom_id,
    primary_key: :classroom_id,
    conditions:  Proc.new { { enrollments: { student_id: student_id } } }