Some slug handling cleanup.

shared/utils/fields.py

@@ -17,17 +17,25 @@ def uniquify_field_value(instance, field_name, value, max_length=None, queryset=
 
     FIXME Doesn't work with model inheritance, where the field is part of the parent class.
     """
-    def get_similar_values(value):
-        return queryset.exclude(pk=instance.pk) \
-            .filter(**{"%s__istartswith" % field_name: value}).values_list(field_name, flat=True)
+    if not queryset:
+        queryset = instance._meta.default_manager.get_queryset()
 
-    if not value:
-        raise ValueError("Cannot uniquify empty value")
-        # TODO Instead get value from instance.field, or use a default value?
+    field = instance._meta.get_field(field_name)
     if not max_length:
         max_length = instance._meta.get_field(field_name).max_length
-    if not queryset:
-        queryset = instance._meta.default_manager.get_queryset()
+    if not value:
+        if field.blank:
+            # Special case: Make sure only one row has ean empty value
+            if queryset.exclude(pk=instance.pk).filter(**{field_name: ''}).exists():
+                raise ValueError("Only one blank (root) entry allowed.")
+            return
+        else:
+            raise ValueError("Cannot uniquify empty value")
+            # TODO Instead get value from instance.field, or use a default value?
+
+    def get_similar_values(value):
+        return queryset.exclude(pk=instance.pk) \
+            .filter(**{"%s__istartswith" % field_name: value}).values_list(field_name, flat=True)
 
     # Find already existing counter
     m = re.match(r'(.+)(-\d+)$', value)