stats: provide migration path for stats calculated in older versions

Few keys were moved from optional to required and we still need to deal
with them being missing.

Fixes #10439
Fixes WEBLATE-5K1

docs/changes.rst

@@ -10,6 +10,7 @@ Not yet released.
 **Bug fixes**
 
 * Database backups compatibility with Alibaba Cloud Database PolarDB.
+* Crash on loading statistics calculated by previous versions.
 
 **Upgrading**
 

weblate/utils/stats.py

@@ -76,6 +76,9 @@
     )
 )
 
+# TODO: Drop in Weblate 5.5
+LEGACY_KEYS = {"unapproved", "unapproved_chars", "unapproved_words", "total_changes"}
+
 SOURCE_MAP = {
     "source_chars": "all_chars",
     "source_words": "all_words",
@@ -213,6 +216,12 @@ def aggregate_get(self, name: str):
             # Handle source_* keys as virtual on translation level for easier aggregation
             if name.startswith("source_"):
                 return self._data[SOURCE_MAP[name]]
+            # Legacy keys were calculated on demand before and are precalculated
+            # since Weblate 5.2, so they are missing on stats calculated before.
+            # Using zero here is most likely a wrong value, but safe and cheap.
+            # TODO: Drop in Weblate 5.5
+            if name in LEGACY_KEYS:
+                return 0
             raise
 
     def __getattr__(self, name: str):
@@ -226,7 +235,7 @@ def __getattr__(self, name: str):
             return self.calculate_percent(name)
 
         if name == "stats_timestamp":
-            # TODO: Drop in Weblate 5.3
+            # TODO: Drop in Weblate 5.5
             # Migration path for legacy stat data
             return self._data.get(name, 0)