fix(unit): add some missing prefetch() calls

The prefetch_bulk() and prefetch_full() do prefetch unit data, but not
parent objects with assumption that these are typically called from
translation.unit_set where such prefetch is not needed. But when it
originates from Unit.objects, it needs to be added.

docs/changes.rst

@@ -10,6 +10,7 @@ Not yet released.
 **Bug fixes**
 
 * Update outdated plural definitions during the database migration.
+* Reduced number of database queries when updating multiple strings.
 
 **Compatibility**
 

weblate/trans/autotranslate.py

@@ -137,7 +137,12 @@ def process_others(self, source: int | None) -> None:
             .filter(source__in=translations.keys())
             .values_list("id", flat=True)
         )
-        units = Unit.objects.filter(pk__in=unit_ids).prefetch_bulk().select_for_update()
+        units = (
+            Unit.objects.filter(pk__in=unit_ids)
+            .prefetch()
+            .prefetch_bulk()
+            .select_for_update()
+        )
         self.progress_steps = len(units)
 
         for pos, unit in enumerate(units):

weblate/trans/models/unit.py

@@ -1475,6 +1475,7 @@ def variants(self) -> Iterable[Unit]:
             return []
         return (
             self.variant.unit_set.filter(translation=self.translation)
+            .prefetch()
             .prefetch_full()
             .order_by("context")
         )
@@ -1647,6 +1648,7 @@ def checksum(self):
     def same_source_units(self) -> UnitQuerySet:
         return (
             Unit.objects.same(self)
+            .prefetch()
             .prefetch_full()
             .filter(
                 translation__component__allow_translation_propagation=True,