Refuse to change multi to single attribute if there are values

Prevent users from accidentally changing multi attributes to single ones
without changing all the attribute values before to contain only one.

Author: kofrezo

serveradmin/serverdb/forms.py

@@ -1,7 +1,8 @@
 from django import forms
 from django.core.exceptions import ValidationError
+from django.db.models.aggregates import Count
 
-from serveradmin.serverdb.models import ServertypeAttribute, Attribute
+from serveradmin.serverdb.models import ServertypeAttribute, Attribute, ServerStringAttribute
 
 
 class ServertypeAdminForm(forms.ModelForm):
@@ -24,8 +25,8 @@ def clean(self):
         # It makes no sense to add inet or supernet attributes to hosts of
         # ip_addr_type null because they would have to be empty anyways.
         inet_attribute = (
-            self.cleaned_data['attribute'].type in ('inet', 'supernet') and
-            self.instance.servertype.ip_addr_type == 'null'
+                self.cleaned_data['attribute'].type in ('inet', 'supernet') and
+                self.instance.servertype.ip_addr_type == 'null'
         )
         if inet_attribute:
             raise ValidationError(
@@ -49,4 +50,12 @@ def clean(self):
         if attr_type == 'inet' and self.cleaned_data.get('multi') is True:
             raise ValidationError('Multi attributes of type inet are not supported!')
 
-        super().clean()
+        if self.cleaned_data.get('multi') is False:
+            any_attrs_have_multiple_values = ServerStringAttribute.get_model(self.instance.type).objects.filter(
+                attribute_id=self.instance.attribute_id).values('server_id').annotate(
+                occurences=Count('server_id')).filter(occurences__gt=1).exists()
+            if any_attrs_have_multiple_values:
+                raise ValidationError(
+                    'Refusing to make attribute type single because one ore more objects still have multiple values!')
+
+        super().clean()