class AddUniqIndexToExerciseAnswer < ActiveRecord::Migration[5.2]
  def change
    # remove_index :exercise_answers, column: [:exercise_question_id, :user_id]
    #
    # change_column_default :exercise_answers, :exercise_choice_id, from: nil, to: -1
    # ExerciseAnswer.where(exercise_choice_id: nil).update_all(exercise_choice_id: -1)

    # sql = %Q(delete from exercise_answers where (exercise_question_id, user_id, exercise_choice_id) in
    #         (select * from (select exercise_question_id, user_id, exercise_choice_id from exercise_answers group by exercise_question_id, user_id, exercise_choice_id having count(*) > 1) a)
    #         and id not in (select * from (select min(id) from exercise_answers group by exercise_question_id, user_id, exercise_choice_id having count(*) > 1 order by id) b))
    # ActiveRecord::Base.connection.execute sql
    #
    # add_index :exercise_answers, [:exercise_question_id, :user_id, :exercise_choice_id], name: 'exercise_choice_index', unique: true


  end
end
