删除空白的答案

db/migrate/20190723152256_delete_contents_is_null_for_challenge_answers.rb

@@ -0,0 +1,6 @@
+class DeleteContentsIsNullForChallengeAnswers < ActiveRecord::Migration[5.2]
+  def change
+    contents = ChallengeAnswer.where("contents = ''")
+    contents.delete_all
+  end
+end