新增：forum_section关系

app/models/forum_section.rb

@@ -0,0 +1,4 @@
+class ForumSection < ApplicationRecord
+  has_many :memos, dependent: :destroy
+
+end

app/models/memo.rb

@@ -45,6 +45,7 @@ class Memo < ApplicationRecord
   include Watchable
 
   belongs_to :author, :class_name => "User", foreign_key: :author_id, counter_cache: true
+  belongs_to :forum_section, counter_cache: true
 
   enum destroy_status: {common: 0, apply_destroy: 1}
 

app/views/api/v1/users/memos/_memo.json.jbuilder

@@ -3,7 +3,7 @@ json.apply_destroy memo.apply_destroy?
 json.content memo.content
 json.cover_url memo.cover_url
 json.forum_section_id memo.forum_section_id
-json.forum_section_title memo.forum_section_title
+json.forum_section_title memo.forum_section&.title
 json.id memo.id
 json.is_fine memo.is_fine
 json.is_original memo.is_original