Fixes: #18336 - Perform Rack object validation of u_height and starting_unit on rack_type if present (#18395)

* Perform Rack object validation of u_height and starting_unit on rack_type if present

* Calculate effective values before doing validation

netbox/dcim/models/racks.py

@@ -374,22 +374,27 @@ class Rack(ContactsMixin, ImageAttachmentsMixin, RackBase):
         if not self._state.adding:
             mounted_devices = Device.objects.filter(rack=self).exclude(position__isnull=True).order_by('position')
 
+            effective_u_height = self.rack_type.u_height if self.rack_type else self.u_height
+            effective_starting_unit = self.rack_type.starting_unit if self.rack_type else self.starting_unit
+
             # Validate that Rack is tall enough to house the highest mounted Device
             if top_device := mounted_devices.last():
-                min_height = top_device.position + top_device.device_type.u_height - self.starting_unit
-                if self.u_height < min_height:
+                min_height = top_device.position + top_device.device_type.u_height - effective_starting_unit
+                if effective_u_height < min_height:
+                    field = 'rack_type' if self.rack_type else 'u_height'
                     raise ValidationError({
-                        'u_height': _(
+                        field: _(
                             "Rack must be at least {min_height}U tall to house currently installed devices."
                         ).format(min_height=min_height)
                     })
 
             # Validate that the Rack's starting unit is less than or equal to the position of the lowest mounted Device
             if last_device := mounted_devices.first():
-                if self.starting_unit > last_device.position:
+                if effective_starting_unit > last_device.position:
+                    field = 'rack_type' if self.rack_type else 'starting_unit'
                     raise ValidationError({
-                        'starting_unit': _("Rack unit numbering must begin at {position} or less to house "
-                                           "currently installed devices.").format(position=last_device.position)
+                        field: _("Rack unit numbering must begin at {position} or less to house "
+                                 "currently installed devices.").format(position=last_device.position)
                     })
 
             # Validate that Rack was assigned a Location of its same site, if applicable