FIX: applies a unique index for accepted_answer_post_id custom field (#83)

db/migrate/20191209095548_ensures_unique_accepted_answer_post_id.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class EnsuresUniqueAcceptedAnswerPostId < ActiveRecord::Migration[5.2]
+  def change
+    execute <<~SQL
+      DELETE FROM topic_custom_fields AS tcf1
+      USING topic_custom_fields AS tcf2
+      WHERE tcf1.id > tcf2.id AND
+         tcf1.topic_id = tcf2.topic_id AND
+         tcf1.name = tcf2.name AND
+         tcf1.name = 'accepted_answer_post_id'
+    SQL
+
+    add_index :topic_custom_fields,
+      :topic_id,
+      name: :idx_topic_custom_fields_accepted_answer,
+      unique: true,
+      where: "name = 'accepted_answer_post_id'"
+  end
+end