Closes #12133: Move any instance mutations inside clean() to save()

netbox/dcim/models/cables.py

@@ -152,8 +152,6 @@ class Cable(PrimaryModel):
         # Validate length and length_unit
         if self.length is not None and not self.length_unit:
             raise ValidationError("Must specify a unit when setting a cable length")
-        elif self.length is None:
-            self.length_unit = ''
 
         if self.pk is None and (not self.a_terminations or not self.b_terminations):
             raise ValidationError("Must define A and B terminations when creating a new cable.")
@@ -187,6 +185,10 @@ class Cable(PrimaryModel):
         else:
             self._abs_length = None
 
+        # Clear length_unit if no length is defined
+        if self.length is None:
+            self.length_unit = ''
+
         super().save(*args, **kwargs)
 
         # Update the private pk used in __str__ in case this is a new object (i.e. just got its pk)