Now, the user can't select as category parent itself or childrens

expenses_manager/expenses/forms.py

@@ -92,10 +92,43 @@ class CategoryForm(forms.ModelForm):
         super().__init__(*args, **kwargs)
         
         if user:
-            self.fields["parent"].queryset = Category.objects.filter(
-                owner=user,
+            queryset = Category.objects.filter(owner=user)
+            
+            if self.instance.pk:
+                queryset = queryset.exclude(
+                    pk__in=self._self_and_descendant_ids(self.instance)
                 )
             
+            self.fields["parent"].queryset = queryset
+    
+    @staticmethod
+    def _self_and_descendant_ids(category):
+        ids = [category.pk]
+        pending = [category.pk]
+        
+        while pending:
+            children = list(
+                Category.objects.filter(parent_id__in=pending)
+                .exclude(pk__in=ids)
+                .values_list("pk", flat=True)
+            )
+            ids.extend(children)
+            pending = children
+        
+        return ids
+    
+    def clean_parent(self):
+        parent = self.cleaned_data.get("parent")
+
+        if parent and self.instance.pk:
+            if parent.pk in self._self_and_descendant_ids(self.instance):
+                raise forms.ValidationError(
+                    "Una categoría no puede ser su propio padre ni depender "
+                    "de una de sus subcategorías."
+                )
+
+        return parent
+
 
 class GoalForm(forms.ModelForm):
     class Meta: