Fixes #3596: Prevent server error when reassigning a device to a new device bay

netbox/dcim/models.py

@@ -2588,6 +2588,16 @@ class DeviceBay(ComponentModel):
         if self.device == self.installed_device:
             raise ValidationError("Cannot install a device into itself.")
 
+        # Check that the installed device is not already installed elsewhere
+        if self.installed_device:
+            current_bay = DeviceBay.objects.filter(installed_device=self.installed_device).first()
+            if current_bay:
+                raise ValidationError({
+                    'installed_device': "Cannot install the specified device; device is already installed in {}".format(
+                        current_bay
+                    )
+                })
+
 
 #
 # Inventory items