donate: Fix portal API calls for non existing project IDs

donate/models.py

@@ -428,15 +428,22 @@ class DonateProjectPage(DonateFormMixin, Page, SubpageMixin, MetadataPageMixin):
         return self.perex
 
     def get_donated_amount(self):
-        key = f"donated_amount_{self.portal_project_id}"
-        amount = cache.get(key)
-        if amount is None:
-            amount = get_donated_amount_from_api(self.portal_project_id)
-            if amount is not None:
-                self.donated_amount = amount
-                self.save()
-                cache.set(key, amount)
-        return self.donated_amount
+        if self.portal_project_id is None:
+            return 0
+        # instance caching for multiple method calls during one request
+        if not hasattr(self, "_donated_amount"):
+            # cache portal API calls (defaults to 5 min)
+            key = f"donated_amount_{self.portal_project_id}"
+            amount = cache.get(key)
+            if amount is None:
+                amount = get_donated_amount_from_api(self.portal_project_id)
+                if amount is not None:
+                    # save amount into database to be used if next API calls fails
+                    self.donated_amount = amount
+                    self.save()
+                    cache.set(key, amount)
+            self._donated_amount = self.donated_amount or 0
+        return self._donated_amount
 
     @property
     def donated_percentage(self):