Fixed #27719 -- Added QuerySet.alias() to allow creating reusable aliases.

QuerySet.alias() allows creating reusable aliases for expressions that
don't need to be selected but are used for filtering, ordering, or as
a part of complex expressions.

Thanks Simon Charette for reviews.

django/db/models/query.py

@@ -1085,6 +1085,16 @@ class QuerySet:
         with extra data or aggregations.
         """
         self._not_support_combined_queries('annotate')
+        return self._annotate(args, kwargs, select=True)
+
+    def alias(self, *args, **kwargs):
+        """
+        Return a query set with added aliases for extra data or aggregations.
+        """
+        self._not_support_combined_queries('alias')
+        return self._annotate(args, kwargs, select=False)
+
+    def _annotate(self, args, kwargs, select=True):
         self._validate_values_are_expressions(args + tuple(kwargs.values()), method_name='annotate')
         annotations = {}
         for arg in args:
@@ -1114,8 +1124,9 @@ class QuerySet:
             if isinstance(annotation, FilteredRelation):
                 clone.query.add_filtered_relation(annotation, alias)
             else:
-                clone.query.add_annotation(annotation, alias, is_summary=False)
-
+                clone.query.add_annotation(
+                    annotation, alias, is_summary=False, select=select,
+                )
         for alias, annotation in clone.query.annotations.items():
             if alias in annotations and annotation.contains_aggregate:
                 if clone._fields is None:

django/db/models/sql/query.py

@@ -1015,11 +1015,14 @@ class Query(BaseExpression):
             alias = seen[int_model] = join_info.joins[-1]
         return alias or seen[None]
 
-    def add_annotation(self, annotation, alias, is_summary=False):
+    def add_annotation(self, annotation, alias, is_summary=False, select=True):
         """Add a single annotation expression to the Query."""
         annotation = annotation.resolve_expression(self, allow_joins=True, reuse=None,
                                                    summarize=is_summary)
-        self.append_annotation_mask([alias])
+        if select:
+            self.append_annotation_mask([alias])
+        else:
+            self.set_annotation_mask(set(self.annotation_select).difference({alias}))
         self.annotations[alias] = annotation
 
     def resolve_expression(self, query, *args, **kwargs):
@@ -1707,6 +1710,11 @@ class Query(BaseExpression):
                 # which is executed as a wrapped subquery if any of the
                 # aggregate() elements reference an existing annotation. In
                 # that case we need to return a Ref to the subquery's annotation.
+                if name not in self.annotation_select:
+                    raise FieldError(
+                        "Cannot aggregate over the '%s' alias. Use annotate() "
+                        "to promote it." % name
+                    )
                 return Ref(name, self.annotation_select[name])
             else:
                 return annotation
@@ -1911,6 +1919,11 @@ class Query(BaseExpression):
                 # For lookups spanning over relationships, show the error
                 # from the model on which the lookup failed.
                 raise
+            elif name in self.annotations:
+                raise FieldError(
+                    "Cannot select the '%s' alias. Use annotate() to promote "
+                    "it." % name
+                )
             else:
                 names = sorted([
                     *get_field_names_from_opts(opts), *self.extra,