Refs #27222 -- Refreshed GeneratedFields values on save() initiated update.

This required implementing UPDATE RETURNING machinery that heavily
borrows from the INSERT one.

Author: charettes

django/db/backends/base/features.py

@@ -38,6 +38,7 @@ class BaseDatabaseFeatures:
     can_use_chunked_reads = True
     can_return_columns_from_insert = False
     can_return_rows_from_bulk_insert = False
+    can_return_rows_from_update = False
     has_bulk_insert = True
     uses_savepoints = True
     can_release_savepoints = False

django/db/backends/oracle/base.py

@@ -243,6 +243,7 @@ def __init__(self, *args, **kwargs):
             "use_returning_into", True
         )
         self.features.can_return_columns_from_insert = use_returning_into
+        self.features.can_return_rows_from_update = use_returning_into
 
     @property
     def is_pool(self):

django/db/backends/oracle/features.py

@@ -19,6 +19,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     has_select_for_update_of = True
     select_for_update_of_column = True
     can_return_columns_from_insert = True
+    can_return_rows_from_update = True
     supports_subqueries_in_group_by = False
     ignores_unnecessary_order_by_in_subqueries = False
     supports_tuple_comparison_against_subquery = False

django/db/backends/postgresql/features.py

@@ -11,6 +11,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     allows_group_by_selected_pks = True
     can_return_columns_from_insert = True
     can_return_rows_from_bulk_insert = True
+    can_return_rows_from_update = True
     has_real_datatype = True
     has_native_uuid_field = True
     has_native_duration_field = True

django/db/backends/sqlite3/features.py

@@ -171,3 +171,7 @@ def can_return_columns_from_insert(self):
     can_return_rows_from_bulk_insert = property(
         operator.attrgetter("can_return_columns_from_insert")
     )
+
+    can_return_rows_from_update = property(
+        operator.attrgetter("can_return_columns_from_insert")
+    )

django/db/models/base.py

@@ -1094,12 +1094,28 @@ def _save_table(
             ]
             forced_update = update_fields or force_update
             pk_val = self._get_pk_val(meta)
-            updated = self._do_update(
-                base_qs, using, pk_val, values, update_fields, forced_update
+            returning_fields = [
+                f
+                for f in meta.local_concrete_fields
+                if (
+                    f.generated
+                    and f.referenced_fields.intersection(non_pks_non_generated)
+                )
+            ]
+            results = self._do_update(
+                base_qs,
+                using,
+                pk_val,
+                values,
+                update_fields,
+                forced_update,
+                returning_fields,
             )
-            if force_update and not updated:
+            if updated := bool(results):
+                self._assign_returned_values(results[0], returning_fields)
+            elif force_update:
                 raise self.NotUpdated("Forced update did not affect any rows.")
-            if update_fields and not updated:
+            elif update_fields:
                 raise self.NotUpdated(
                     "Save with update_fields did not affect any rows."
                 )
@@ -1131,11 +1147,19 @@ def _save_table(
                 cls._base_manager, using, fields, returning_fields, raw
             )
             if results:
-                for value, field in zip(results[0], returning_fields):
-                    setattr(self, field.attname, value)
+                self._assign_returned_values(results[0], returning_fields)
         return updated
 
-    def _do_update(self, base_qs, using, pk_val, values, update_fields, forced_update):
+    def _do_update(
+        self,
+        base_qs,
+        using,
+        pk_val,
+        values,
+        update_fields,
+        forced_update,
+        returning_fields,
+    ):
         """
         Try to update the model. Return True if the model was updated (if an
         update query was done and a matching row was found in the DB).
@@ -1147,22 +1171,23 @@ def _do_update(self, base_qs, using, pk_val, values, update_fields, forced_updat
             # case we just say the update succeeded. Another case ending up
             # here is a model with just PK - in that case check that the PK
             # still exists.
-            return update_fields is not None or filtered.exists()
+            if update_fields is not None or filtered.exists():
+                return [()]
+            return []
         if self._meta.select_on_save and not forced_update:
-            return (
-                filtered.exists()
-                and
-                # It may happen that the object is deleted from the DB right
-                # after this check, causing the subsequent UPDATE to return
-                # zero matching rows. The same result can occur in some rare
-                # cases when the database returns zero despite the UPDATE being
-                # executed successfully (a row is matched and updated). In
-                # order to distinguish these two cases, the object's existence
-                # in the database is again checked for if the UPDATE query
-                # returns 0.
-                (filtered._update(values) > 0 or filtered.exists())
-            )
-        return filtered._update(values) > 0
+            # It may happen that the object is deleted from the DB right after
+            # this check, causing the subsequent UPDATE to return zero matching
+            # rows. The same result can occur in some rare cases when the
+            # database returns zero despite the UPDATE being executed
+            # successfully (a row is matched and updated). In order to
+            # distinguish these two cases, the object's existence in the
+            # database is again checked for if the UPDATE query returns 0.
+            if not filtered.exists():
+                return []
+            if results := filtered._update(values, returning_fields):
+                return results
+            return [()] if filtered.exists() else []
+        return filtered._update(values, returning_fields)
 
     def _do_insert(self, manager, using, fields, returning_fields, raw):
         """
@@ -1177,6 +1202,10 @@ def _do_insert(self, manager, using, fields, returning_fields, raw):
             raw=raw,
         )
 
+    def _assign_returned_values(self, returned_values, returning_fields):
+        for value, field in zip(returned_values, returning_fields):
+            setattr(self, field.attname, value)
+
     def _prepare_related_fields_for_save(self, operation_name, fields=None):
         # Ensure that a model instance without a PK hasn't been assigned to
         # a ForeignKey, GenericForeignKey or OneToOneField on this model. If

django/db/models/fields/generated.py

@@ -66,6 +66,16 @@ def generated_sql(self, connection):
             sql = f"CASE WHEN {sql} THEN 1 ELSE 0 END"
         return sql, params
 
+    @cached_property
+    def referenced_fields(self):
+        resolved_expression = self.expression.resolve_expression(
+            self._query, allow_joins=False
+        )
+        referenced_fields = []
+        for col in self._query._gen_cols([resolved_expression]):
+            referenced_fields.append(col.target)
+        return frozenset(referenced_fields)
+
     def check(self, **kwargs):
         databases = kwargs.get("databases") or []
         errors = [

django/db/models/query.py

@@ -1306,7 +1306,7 @@ async def aupdate(self, **kwargs):
 
     aupdate.alters_data = True
 
-    def _update(self, values):
+    def _update(self, values, returning_fields=None):
         """
         A version of update() that accepts field objects instead of field
         names. Used primarily for model saving and not intended for use by
@@ -1320,7 +1320,9 @@ def _update(self, values):
         # Clear any annotations so that they won't be present in subqueries.
         query.annotations = {}
         self._result_cache = None
-        return query.get_compiler(self.db).execute_sql(ROW_COUNT)
+        if returning_fields is None:
+            return query.get_compiler(self.db).execute_sql(ROW_COUNT)
+        return query.get_compiler(self.db).execute_returning_sql(returning_fields)
 
     _update.alters_data = True
     _update.queryset_only = False

django/db/models/sql/compiler.py

@@ -2020,6 +2020,9 @@ def as_sql(self):
 
 
 class SQLUpdateCompiler(SQLCompiler):
+    returning_fields = None
+    returning_params = ()
+
     def as_sql(self):
         """
         Create the SQL for this query. Return the SQL string and list of
@@ -2087,6 +2090,15 @@ def as_sql(self):
             params = []
         else:
             result.append("WHERE %s" % where)
+        if self.returning_fields:
+            # Skip empty r_sql to allow subclasses to customize behavior for
+            # 3rd party backends. Refs #19096.
+            r_sql, self.returning_params = self.connection.ops.returning_columns(
+                self.returning_fields
+            )
+            if r_sql:
+                result.append(r_sql)
+                params.extend(self.returning_params)
         return " ".join(result), tuple(update_params + params)
 
     def execute_sql(self, result_type):
@@ -2110,6 +2122,38 @@ def execute_sql(self, result_type):
                 is_empty = False
         return row_count
 
+    def execute_returning_sql(self, returning_fields):
+        """
+        Execute the specified update and return rows of the returned columns
+        associated with the specified returning_field if the backend supports
+        it.
+        """
+        if self.query.get_related_updates():
+            raise NotImplementedError(
+                "Update returning is not implemented for queries with related updates."
+            )
+
+        if (
+            not returning_fields
+            or not self.connection.features.can_return_rows_from_update
+        ):
+            row_count = self.execute_sql(ROW_COUNT)
+            return [()] * row_count
+
+        self.returning_fields = returning_fields
+        with self.connection.cursor() as cursor:
+            sql, params = self.as_sql()
+            cursor.execute(sql, params)
+            rows = self.connection.ops.fetch_returned_rows(
+                cursor, self.returning_params
+            )
+        opts = self.query.get_meta()
+        cols = [field.get_col(opts.db_table) for field in self.returning_fields]
+        converters = self.get_converters(cols)
+        if converters:
+            rows = self.apply_converters(rows, converters)
+        return list(rows)
+
     def pre_sql_setup(self):
         """
         If the update depends on results from other tables, munge the "where"

docs/ref/models/fields.txt

@@ -1315,12 +1315,6 @@ materialized view.
     PostgreSQL only supports persisted columns. Oracle only supports virtual
     columns.
 
-.. admonition:: Refresh the data
-
-    Since the database computes the value, the object must be reloaded to
-    access the new value after :meth:`~Model.save`, for example, by using
-    :meth:`~Model.refresh_from_db`.
-
 .. admonition:: Database limitations
 
     There are many database-specific restrictions on generated fields that
@@ -1338,6 +1332,12 @@ materialized view.
 .. _PostgreSQL: https://www.postgresql.org/docs/current/ddl-generated-columns.html
 .. _SQLite: https://www.sqlite.org/gencol.html#limitations
 
+.. versionchanged:: 6.0
+
+    ``GeneratedField``\s are now automatically refreshed from the database on
+    backends that support it (SQLite, PostgreSQL, and Oracle) and marked as
+    deferred otherwise.
+
 ``GenericIPAddressField``
 -------------------------