+class MerchandisePurchase < Purchase
+ belongs_to :merchandise
+ belongs_to :customer
+
+ validates_presence_of :merchandise_id
+ validates_presence_of :quantity
+
+ validates_numericality_of :merchandise_id
+ validates_numericality_of :quantity
+
+ protected
+ def validate
+ errors.add(:merchandise_id, "does not exist in the database") if merchandise.nil?
+ #errors.add(:quantity, "must be 1 or greater") if quantity < 1
+ end
+end