:learners, :full_name
  :learners, :email, unique: true
  :quizzes, :name
  :questions, :title
  # Assuming people are learning to pass, so there will be much less incorrect answers, hence partial index on them is reasonable
  :possible_answers, :value, where: "value = 'Possible Answer Incorrect'"
  # As above, partial index is reasonable on not correct answers(as soon as I correctly understood meaning of these columns)
  :possible_answers, :correct, where: "correct = FALSE"
  # change autogenerated :answers, :possible_answer_id to (since there are only two possible values of :possible_answer):
  :answers, [:possible_answer_id, :learner_id]