Create Appendable model

app/models/appendable.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+class Appendable < ApplicationRecord
+  belongs_to :parent, polymorphic: true
+  belongs_to :user
+end

db/migrate/20220127212925_create_appendables.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+class CreateAppendables < ActiveRecord::Migration[6.1]
+  def up
+    create_table :appendables do |t|
+      t.string :type, null: false
+      t.bigint :user_id, null: false
+      t.bigint :parent_id, null: false
+      t.string :parent_type, null: false
+      t.text :content
+
+      t.timestamps
+    end
+
+    change_column :appendables, :id, :bigint
+    add_index :appendables, %i[parent_id parent_type]
+    add_index :appendables, %i[user_id created_at]
+  end
+
+  def down
+    drop_table :appendables
+  end
+end