Refs #27149 -- Moved subquery expression resolving to Query.

This makes Subquery a thin wrapper over Query and makes sure it respects
the Expression source expression API by accepting the same number of
expressions as it returns. Refs #30188.

It also makes OuterRef usable in Query without Subquery wrapping. This
should allow Query's internals to more easily perform subquery push downs
during split_exclude(). Refs #21703.

django/db/models/expressions.py

@@ -1002,70 +1002,28 @@ class Subquery(Expression):
         self.extra = extra
         super().__init__(output_field)
 
+    def get_source_expressions(self):
+        return [self.query]
+
+    def set_source_expressions(self, exprs):
+        self.query = exprs[0]
+
     def _resolve_output_field(self):
-        if len(self.query.select) == 1:
-            return self.query.select[0].field
-        return super()._resolve_output_field()
+        return self.query.output_field
 
     def copy(self):
         clone = super().copy()
         clone.query = clone.query.clone()
         return clone
 
-    def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
-        clone = self.copy()
-        clone.is_summary = summarize
-        clone.query.bump_prefix(query)
-
-        # Need to recursively resolve these.
-        def resolve_all(child):
-            if hasattr(child, 'children'):
-                [resolve_all(_child) for _child in child.children]
-            if hasattr(child, 'rhs'):
-                child.rhs = resolve(child.rhs)
-
-        def resolve(child):
-            if hasattr(child, 'resolve_expression'):
-                resolved = child.resolve_expression(
-                    query=query, allow_joins=allow_joins, reuse=reuse,
-                    summarize=summarize, for_save=for_save,
-                )
-                # Add table alias to the parent query's aliases to prevent
-                # quoting.
-                if hasattr(resolved, 'alias') and resolved.alias != resolved.target.model._meta.db_table:
-                    clone.query.external_aliases.add(resolved.alias)
-                return resolved
-            return child
-
-        resolve_all(clone.query.where)
-
-        for key, value in clone.query.annotations.items():
-            if isinstance(value, Subquery):
-                clone.query.annotations[key] = resolve(value)
-
-        return clone
-
-    def get_source_expressions(self):
-        return [
-            x for x in [
-                getattr(expr, 'lhs', None)
-                for expr in self.query.where.children
-            ] if x
-        ]
-
-    def relabeled_clone(self, change_map):
-        clone = self.copy()
-        clone.query = clone.query.relabeled_clone(change_map)
-        clone.query.external_aliases.update(
-            alias for alias in change_map.values()
-            if alias not in clone.query.alias_map
-        )
-        return clone
+    @property
+    def external_aliases(self):
+        return self.query.external_aliases
 
     def as_sql(self, compiler, connection, template=None, **extra_context):
         connection.ops.check_expression_support(self)
         template_params = {**self.extra, **extra_context}
-        template_params['subquery'], sql_params = self.query.get_compiler(connection=connection).as_sql()
+        template_params['subquery'], sql_params = self.query.as_sql(compiler, connection)
 
         template = template or template_params.get('template', self.template)
         sql = template % template_params

django/db/models/sql/query.py

@@ -21,7 +21,7 @@ from django.core.exceptions import (
 from django.db import DEFAULT_DB_ALIAS, NotSupportedError, connections
 from django.db.models.aggregates import Count
 from django.db.models.constants import LOOKUP_SEP
-from django.db.models.expressions import Col, F, Ref, SimpleCol
+from django.db.models.expressions import BaseExpression, Col, F, Ref, SimpleCol
 from django.db.models.fields import Field
 from django.db.models.fields.related_lookups import MultiColSource
 from django.db.models.lookups import Lookup
@@ -139,7 +139,7 @@ class RawQuery:
         self.cursor.execute(self.sql, params)
 
 
-class Query:
+class Query(BaseExpression):
     """A single SQL query."""
 
     alias_prefix = 'T'
@@ -231,6 +231,13 @@ class Query:
         self.explain_format = None
         self.explain_options = {}
 
+    @property
+    def output_field(self):
+        if len(self.select) == 1:
+            return self.select[0].field
+        elif len(self.annotation_select) == 1:
+            return next(iter(self.annotation_select.values())).output_field
+
     @property
     def has_select_fields(self):
         return bool(self.select or self.annotation_select_mask or self.extra_select_mask)
@@ -862,7 +869,7 @@ class Query:
             # No clashes between self and outer query should be possible.
             return
 
-        local_recursion_limit = 127  # explicitly avoid infinite loop
+        local_recursion_limit = 67  # explicitly avoid infinite loop
         for pos, prefix in enumerate(prefix_gen()):
             if prefix not in self.subq_aliases:
                 self.alias_prefix = prefix
@@ -997,6 +1004,21 @@ class Query:
                 not self.distinct_fields and
                 not self.select_for_update):
             clone.clear_ordering(True)
+        clone.where.resolve_expression(query, *args, **kwargs)
+        for key, value in clone.annotations.items():
+            resolved = value.resolve_expression(query, *args, **kwargs)
+            if hasattr(resolved, 'external_aliases'):
+                resolved.external_aliases.update(clone.alias_map)
+            clone.annotations[key] = resolved
+        # Outer query's aliases are considered external.
+        clone.external_aliases.update(
+            alias for alias, table in query.alias_map.items()
+            if (
+                isinstance(table, Join) and table.join_field.related_model._meta.db_table != alias
+            ) or (
+                isinstance(table, BaseTable) and table.table_name != table.table_alias
+            )
+        )
         return clone
 
     def as_sql(self, compiler, connection):

django/db/models/sql/where.py

@@ -183,8 +183,23 @@ class WhereNode(tree.Node):
     def is_summary(self):
         return any(child.is_summary for child in self.children)
 
+    @staticmethod
+    def _resolve_rhs(rhs, query, *args, **kwargs):
+        if hasattr(rhs, 'resolve_expression'):
+            rhs = rhs.resolve_expression(query, *args, **kwargs)
+        return rhs
+
+    @classmethod
+    def _resolve_node(cls, node, query, *args, **kwargs):
+        if hasattr(node, 'children'):
+            for child in node.children:
+                cls._resolve_node(child, query, *args, **kwargs)
+        if hasattr(node, 'rhs'):
+            node.rhs = cls._resolve_rhs(node.rhs, query, *args, **kwargs)
+
     def resolve_expression(self, *args, **kwargs):
         clone = self.clone()
+        clone._resolve_node(clone, *args, **kwargs)
         clone.resolved = True
         return clone
 

tests/queries/tests.py

@@ -402,7 +402,7 @@ class Queries1Tests(TestCase):
 
     def test_avoid_infinite_loop_on_too_many_subqueries(self):
         x = Tag.objects.filter(pk=1)
-        local_recursion_limit = 127
+        local_recursion_limit = 67
         msg = 'Maximum recursion depth exceeded: too many subqueries.'
         with self.assertRaisesMessage(RuntimeError, msg):
             for i in range(local_recursion_limit * 2):