Refs #35356 -- Clarified select related with masked field logic.

By always including related objects in the select mask via adjusting the
defer logic (_get_defer_select_mask()), it becomes possible for
select_related_descend() to treat forward and reverse relationships
indistinctively.

This work also simplifies and adds comments to
select_related_descend() to make it easier to understand.

django/db/models/query_utils.py

@@ -340,38 +340,37 @@ class RegisterLookupMixin:
     _unregister_class_lookup = classmethod(_unregister_class_lookup)
 
 
-def select_related_descend(field, restricted, requested, select_mask, reverse=False):
+def select_related_descend(field, restricted, requested, select_mask):
     """
-    Return True if this field should be used to descend deeper for
-    select_related() purposes. Used by both the query construction code
-    (compiler.get_related_selections()) and the model instance creation code
-    (compiler.klass_info).
+    Return whether `field` should be used to descend deeper for
+    `select_related()` purposes.
 
     Arguments:
-     * field - the field to be checked
-     * restricted - a boolean field, indicating if the field list has been
-       manually restricted using a requested clause)
-     * requested - The select_related() dictionary.
-     * select_mask - the dictionary of selected fields.
-     * reverse - boolean, True if we are checking a reverse select related
+     * `field` - the field to be checked. Can be either a `Field` or
+       `ForeignObjectRel` instance.
+     * `restricted` - a boolean field, indicating if the field list has been
+       manually restricted using a select_related() clause.
+     * `requested` - the select_related() dictionary.
+     * `select_mask` - the dictionary of selected fields.
     """
+    # Only relationships can be descended.
     if not field.remote_field:
         return False
-    if field.remote_field.parent_link and not reverse:
+    # Forward MTI parent links should not be explicitly descended as they are
+    # always JOIN'ed against (unless excluded by `select_mask`).
+    if getattr(field.remote_field, "parent_link", False):
         return False
-    if restricted:
-        if reverse and field.related_query_name() not in requested:
-            return False
-        if not reverse and field.name not in requested:
-            return False
-    if not restricted and field.null:
+    # When `select_related()` is used without a `*requested` mask all
+    # relationships are descended unless they are nullable.
+    if not restricted:
+        return not field.null
+    # When `select_related(*requested)` is used only fields that are part of
+    # `requested` should be descended.
+    if field.name not in requested:
         return False
-    if (
-        restricted
-        and select_mask
-        and field.name in requested
-        and field not in select_mask
-    ):
+    # Prevent invalid usages of `select_related()` and `only()`/`defer()` such
+    # as `select_related("a").only("b")` and `select_related("a").defer("a")`.
+    if select_mask and field not in select_mask:
         raise FieldError(
             f"Field {field.model._meta.object_name}.{field.name} cannot be both "
             "deferred and traversed using select_related at the same time."

django/db/models/sql/compiler.py

@@ -1259,11 +1259,10 @@ class SQLCompiler:
             ]
             for related_object, related_field, model in related_fields:
                 if not select_related_descend(
-                    related_field,
+                    related_object,
                     restricted,
                     requested,
                     select_mask,
-                    reverse=True,
                 ):
                     continue
 
@@ -1280,7 +1279,7 @@ class SQLCompiler:
                     "model": model,
                     "field": related_field,
                     "reverse": True,
-                    "local_setter": related_field.remote_field.set_cached_value,
+                    "local_setter": related_object.set_cached_value,
                     "remote_setter": related_field.set_cached_value,
                     "from_parent": from_parent,
                 }
@@ -1296,7 +1295,7 @@ class SQLCompiler:
                     select_fields.append(len(select))
                     select.append((col, None))
                 klass_info["select_fields"] = select_fields
-                next = requested.get(related_field.related_query_name(), {})
+                next = requested.get(related_field_name, {})
                 next_klass_infos = self.get_related_selections(
                     select,
                     related_select_mask,

django/db/models/sql/query.py

@@ -791,44 +791,44 @@ class Query(BaseExpression):
         if select_mask is None:
             select_mask = {}
         select_mask[opts.pk] = {}
-        # All concrete fields that are not part of the defer mask must be
-        # loaded. If a relational field is encountered it gets added to the
-        # mask for it be considered if `select_related` and the cycle continues
-        # by recursively calling this function.
-        for field in opts.concrete_fields:
+        # All concrete fields and related objects that are not part of the
+        # defer mask must be included. If a relational field is encountered it
+        # gets added to the mask for it be considered if `select_related` and
+        # the cycle continues by recursively calling this function.
+        for field in opts.concrete_fields + opts.related_objects:
             field_mask = mask.pop(field.name, None)
-            field_att_mask = mask.pop(field.attname, None)
+            field_att_mask = None
+            if field_attname := getattr(field, "attname", None):
+                field_att_mask = mask.pop(field_attname, None)
             if field_mask is None and field_att_mask is None:
                 select_mask.setdefault(field, {})
             elif field_mask:
                 if not field.is_relation:
                     raise FieldError(next(iter(field_mask)))
+                # Virtual fields such as many-to-many and generic foreign keys
+                # cannot be effectively deferred. Historically, they were
+                # allowed to be passed to QuerySet.defer(). Ignore such field
+                # references until a layer of validation at mask alteration
+                # time is eventually implemented.
+                if field.many_to_many:
+                    continue
                 field_select_mask = select_mask.setdefault(field, {})
-                related_model = field.remote_field.model._meta.concrete_model
+                related_model = field.related_model._meta.concrete_model
                 self._get_defer_select_mask(
                     related_model._meta, field_mask, field_select_mask
                 )
-        # Remaining defer entries must be references to reverse relationships.
-        # The following code is expected to raise FieldError if it encounters
-        # a malformed defer entry.
+        # Remaining defer entries must be references to filtered relations
+        # otherwise they are surfaced as missing field errors.
         for field_name, field_mask in mask.items():
             if filtered_relation := self._filtered_relations.get(field_name):
                 relation = opts.get_field(filtered_relation.relation_name)
                 field_select_mask = select_mask.setdefault((field_name, relation), {})
+                related_model = relation.related_model._meta.concrete_model
+                self._get_defer_select_mask(
+                    related_model._meta, field_mask, field_select_mask
+                )
             else:
-                relation = opts.get_field(field_name)
-                # While virtual fields such as many-to-many and generic foreign
-                # keys cannot be effectively deferred we've historically
-                # allowed them to be passed to QuerySet.defer(). Ignore such
-                # field references until a layer of validation at mask
-                # alteration time will be implemented eventually.
-                if not hasattr(relation, "field"):
-                    continue
-                field_select_mask = select_mask.setdefault(relation, {})
-            related_model = relation.related_model._meta.concrete_model
-            self._get_defer_select_mask(
-                related_model._meta, field_mask, field_select_mask
-            )
+                opts.get_field(field_name)
         return select_mask
 
     def _get_only_select_mask(self, opts, mask, select_mask=None):