add a customer property to the Warranty and Backup model

network_inventory/inventory/models/backup.py

@@ -30,6 +30,10 @@ class Backup(models.Model):
     def __str__(self):
         return str(self.name)
 
+    @property
+    def customer(self):
+        return self.computer.customer
+
 
 class TargetDevice(models.Model):
     device = models.ForeignKey(Computer, models.SET_NULL, blank=True,

network_inventory/inventory/models/warranty.py

@@ -23,3 +23,7 @@ class Warranty(models.Model):
 
     class Meta:
         verbose_name_plural = "Warranties"
+
+    @property
+    def customer(self):
+        return self.device.customer