迁移调整

db/migrate/20191203124903_add_index_to_exercise_choice.rb

@@ -1,6 +1,6 @@
 class AddIndexToExerciseChoice < ActiveRecord::Migration[5.2]
   def change
-    remove_index :exercise_answers, column: [:exercise_question_id, :user_id]
+    remove_index :exercise_answers, name: :index_on_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)