diff --git a/app/models/message.rb b/app/models/message.rb
index b058bad..62c9b30 100644
--- a/app/models/message.rb
+++ b/app/models/message.rb
@@ -63,6 +63,8 @@ def from_mail(mail, list, list_seq)
       end
     end
 
+    self.yyyymm = self.published_at.strftime('%Y%m')
+
     self
   end
 
diff --git a/db/migrate/20251220051221_add_yyyymm_to_messages.rb b/db/migrate/20251220051221_add_yyyymm_to_messages.rb
new file mode 100644
index 0000000..dc10ecf
--- /dev/null
+++ b/db/migrate/20251220051221_add_yyyymm_to_messages.rb
@@ -0,0 +1,5 @@
+class AddYyyymmToMessages < ActiveRecord::Migration[8.1]
+  def change
+    add_column :messages, :yyyymm, :integer
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index fe9c61d..9acb8f4 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,37 +10,37 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2025_10_22_024234) do
+ActiveRecord::Schema[8.1].define(version: 2025_12_20_051221) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_catalog.plpgsql"
   enable_extension "pg_trgm"
 
   create_table "active_storage_attachments", force: :cascade do |t|
-    t.string "name", null: false
-    t.string "record_type", null: false
-    t.bigint "record_id", null: false
     t.bigint "blob_id", null: false
     t.datetime "created_at", null: false
+    t.string "name", null: false
+    t.bigint "record_id", null: false
+    t.string "record_type", null: false
     t.index ["blob_id"], name: "index_active_storage_attachments_on_blob_id"
     t.index ["record_type", "record_id", "name", "blob_id"], name: "index_active_storage_attachments_uniqueness", unique: true
   end
 
   create_table "active_storage_blobs", force: :cascade do |t|
-    t.string "key", null: false
-    t.string "filename", null: false
-    t.string "content_type"
-    t.text "metadata"
-    t.string "service_name", null: false
     t.bigint "byte_size", null: false
     t.string "checksum"
+    t.string "content_type"
     t.datetime "created_at", null: false
+    t.string "filename", null: false
+    t.string "key", null: false
+    t.text "metadata"
+    t.string "service_name", null: false
     t.index ["key"], name: "index_active_storage_blobs_on_key", unique: true
   end
 
   create_table "active_storage_file_blobs", force: :cascade do |t|
-    t.string "key"
-    t.binary "data"
     t.datetime "created_at", null: false
+    t.binary "data"
+    t.string "key"
     t.datetime "updated_at", null: false
     t.index ["key"], name: "index_active_storage_file_blobs_on_key", unique: true
   end
@@ -52,17 +52,18 @@
   end
 
   create_table "messages", force: :cascade do |t|
-    t.string "subject"
-    t.string "from"
     t.text "body"
     t.datetime "created_at", null: false
-    t.datetime "updated_at", null: false
+    t.string "from"
+    t.text "html_body"
     t.integer "list_id"
     t.integer "list_seq"
-    t.timestamptz "published_at"
     t.string "message_id_header"
     t.integer "parent_id"
-    t.text "html_body"
+    t.timestamptz "published_at"
+    t.string "subject"
+    t.datetime "updated_at", null: false
+    t.integer "yyyymm"
     t.index ["body"], name: "index_messages_on_body", opclass: :gin_trgm_ops, using: :gin
     t.index ["list_id", "list_seq"], name: "index_messages_on_list_id_and_list_seq", unique: true
     t.index ["message_id_header"], name: "index_messages_on_message_id_header"