Make StockEvent a base class for Delivery and StockTaking

This helps to share code between the two entities and allows easier
extensions in the future.

db/migrate/20181204070000_create_stock_events.rb

@@ -0,0 +1,41 @@
+class CreateStockEvents < ActiveRecord::Migration
+  class StockEvent < ActiveRecord::Base; end
+  class StockTaking < ActiveRecord::Base; end
+
+  def change
+    rename_table :deliveries, :stock_events
+    rename_column :stock_changes, :delivery_id, :stock_event_id
+    add_column :stock_events, :type, :string, default: 'Delivery', null: false
+
+    reversible do |dir|
+      dir.up do
+        change_column_default :stock_events, :type, nil
+
+        diff = [StockEvent.maximum(:id) + 1 - StockTaking.minimum(:id), 0].max
+
+        execute "UPDATE stock_changes SET stock_event_id = stock_taking_id + #{diff}
+          WHERE stock_taking_id IS NOT NULL"
+        execute "INSERT INTO stock_events (type, id, date, note, created_at)
+          SELECT 'StockTaking', id + #{diff}, date, note, created_at FROM stock_takings"
+
+        remove_column :stock_changes, :stock_taking_id
+        drop_table :stock_takings
+      end
+
+      dir.down do
+        create_table :stock_takings do |t|
+          t.date :date
+          t.text :note
+          t.datetime :created_at
+        end
+        add_column :stock_changes, :stock_taking_id, :integer
+
+        execute "INSERT INTO stock_takings (id, date, note, created_at)
+          SELECT id, date, note, created_at FROM stock_events WHERE type = 'StockTaking'"
+        execute "DELETE FROM stock_events WHERE type = 'StockTaking'"
+        execute "UPDATE stock_changes SET stock_taking_id = stock_event_id, stock_event_id = NULL
+          WHERE stock_event_id IN (SELECT id FROM stock_takings)"
+      end
+    end
+  end
+end