迁移调整

db/migrate/20191203122643_add_index_to_exercise_answers.rb

@@ -1,5 +1,5 @@
 class AddIndexToExerciseAnswers < ActiveRecord::Migration[5.2]
   def change
-    add_index :exercise_answers, column: [:exercise_question_id, :user_id]
+    add_index :exercise_answers, [:exercise_question_id, :user_id], name: "index_on_question_id_user_id"
   end
 end