#14 共通のサムネとタイトルの取得は完成

backend/app/models/post.rb

@@ -1,3 +1,6 @@
+require 'mini_magick'
+
+
 class Post < ApplicationRecord
   belongs_to :parent, class_name: 'Post', optional: true, foreign_key: 'parent_id'
   belongs_to :uploaded_user, class_name: 'User', optional: true
@@ -12,4 +15,15 @@ class Post < ApplicationRecord
                                           thumbnail, only_path: false) :
                                       nil })
   end
+
+  def resized_thumbnail!
+    return unless thumbnail.attached?
+
+    image = MiniMagick::Image.read(thumbnail.download)
+    image.resize '180x180'
+    thumbnail.purge
+    thumbnail.attach(io:            File.open(image.path),
+                     filename:      'resized_thumbnail.jpg',
+                     content_type:  'image/jpeg')
+  end
 end