fix(change): cache project renaming lookup

There is no index on the old field so this ends up being index based
scan which is expensive. So better to cache the few results we have and
reuse it.

Fixes WEBLATE-7QC

weblate/middleware.py

@@ -112,16 +112,8 @@ def fixup_project(self, slug, request):
         except Project.MultipleObjectsReturned:
             return None
         except Project.DoesNotExist:
-            try:
-                project = (
-                    Change.objects.filter(
-                        action=Change.ACTION_RENAME_PROJECT,
-                        old=slug,
-                    )
-                    .order()[0]
-                    .project
-                )
-            except IndexError:
+            project = Change.objects.lookup_project_rename(slug)
+            if project is None:
                 return None
 
         request.user.check_access(project)

weblate/trans/models/change.py

@@ -31,6 +31,9 @@
     from weblate.trans.models import Translation
 
 
+CHANGE_PROJECT_LOOKUP_KEY = "change:project-lookup"
+
+
 class ChangeQuerySet(models.QuerySet["Change"]):
     def content(self, prefetch=False):
         """Return queryset with content changes."""
@@ -206,6 +209,25 @@ def filter_projects(self, user):
             return self
         return self.filter(project__in=user.allowed_projects)
 
+    def lookup_project_rename(self, name: str) -> Project:
+        lookup = cache.get(CHANGE_PROJECT_LOOKUP_KEY)
+        if lookup is None:
+            lookup = self.generate_project_rename_lookup()
+        if name not in lookup:
+            return None
+        try:
+            return Project.objects.get(pk=lookup[name])
+        except Project.DoesNotExist:
+            return None
+
+    def generate_project_rename_lookup(self) -> dict[str, int]:
+        lookup = {}
+        for change in self.filter(action=Change.ACTION_RENAME_PROJECT).order():
+            if change.old not in lookup:
+                lookup[change.old] = change.project_id
+        cache.set(CHANGE_PROJECT_LOOKUP_KEY, lookup, 3600 * 24 * 7)
+        return lookup
+
 
 class ChangeManager(models.Manager["Change"]):
     def create(self, *, user=None, **kwargs):
@@ -641,6 +663,9 @@ def save(self, *args, **kwargs) -> None:
             # Make sure stats is updated at the end of transaction
             self.translation.invalidate_cache()
 
+        if self.action == Change.ACTION_RENAME_PROJECT:
+            Change.objects.generate_project_rename_lookup()
+
     def get_absolute_url(self):
         """Return link either to unit or translation."""
         if self.unit is not None: