class AddUniqIndexToCourseMember < ActiveRecord::Migration[5.2]
  def change
    # sql = %Q(delete from course_members where (user_id, course_id, role) in
    #         (select * from (select user_id, course_id, role from course_members group by user_id, course_id, role having count(*) > 1) a)
    #         and id not in (select * from (select min(id) from course_members group by user_id, course_id, role having count(*) > 1 order by id) b))
    # ActiveRecord::Base.connection.execute sql
    #
    # add_index :course_members, [:user_id, :course_id, :role], unique: true, name: "index_user_id_course_id_role"
  end
end
