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.

app/models/delivery.rb

@@ -1,12 +1,11 @@
-class Delivery < ApplicationRecord
+class Delivery < StockEvent
 
   belongs_to :supplier
   belongs_to :invoice
-  has_many :stock_changes, -> { includes(:stock_article).order('articles.name ASC') }, :dependent => :destroy
 
   scope :recent, -> { order('created_at DESC').limit(10) }
 
-  validates_presence_of :supplier_id, :date
+  validates_presence_of :supplier_id
   validate :stock_articles_must_be_unique
 
   accepts_nested_attributes_for :stock_changes, :allow_destroy => :true

app/models/stock_change.rb

@@ -1,7 +1,7 @@
 class StockChange < ApplicationRecord
-  belongs_to :delivery
+  belongs_to :delivery, foreign_key: 'stock_event_id'
   belongs_to :order
-  belongs_to :stock_taking
+  belongs_to :stock_taking, foreign_key: 'stock_event_id'
   belongs_to :stock_article
 
   validates_presence_of :stock_article_id, :quantity

app/models/stock_event.rb

@@ -0,0 +1,8 @@
+class StockEvent < ApplicationRecord
+
+  has_many :stock_changes, dependent: :destroy
+  has_many :stock_articles, through: :stock_changes
+
+  validates_presence_of :date
+
+end

app/models/stock_taking.rb

@@ -1,10 +1,4 @@
-class StockTaking < ApplicationRecord
-
-  has_many :stock_changes, :dependent => :destroy
-  has_many :stock_articles, :through => :stock_changes
-
-  validates_presence_of :date
-
+class StockTaking < StockEvent
   def stock_change_attributes=(stock_change_attributes)
     for attributes in stock_change_attributes
       stock_changes.build(attributes) unless attributes[:quantity].to_i == 0