Fixed #28586 -- Added model field fetch modes.

May your database queries be much reduced with minimal effort.

co-authored-by: Andreas Pelme <andreas@pelme.se>
co-authored-by: Simon Charette <charette.s@gmail.com>
co-authored-by: Jacob Walls <jacobtylerwalls@gmail.com>

django/contrib/contenttypes/fields.py

@@ -16,6 +16,7 @@ from django.db.models.fields.related import (
     ReverseManyToOneDescriptor,
     lazy_related_operation,
 )
+from django.db.models.query import prefetch_related_objects
 from django.db.models.query_utils import PathInfo
 from django.db.models.sql import AND
 from django.db.models.sql.where import WhereNode
@@ -253,6 +254,15 @@ class GenericForeignKeyDescriptor:
                 return rel_obj
             else:
                 rel_obj = None
+
+        instance._state.fetch_mode.fetch(self, instance)
+        return self.field.get_cached_value(instance)
+
+    def fetch_one(self, instance):
+        f = self.field.model._meta.get_field(self.field.ct_field)
+        ct_id = getattr(instance, f.attname, None)
+        pk_val = getattr(instance, self.field.fk_field)
+        rel_obj = None
         if ct_id is not None:
             ct = self.field.get_content_type(id=ct_id, using=instance._state.db)
             try:
@@ -262,7 +272,11 @@ class GenericForeignKeyDescriptor:
             except ObjectDoesNotExist:
                 pass
         self.field.set_cached_value(instance, rel_obj)
-        return rel_obj
+
+    def fetch_many(self, instances):
+        is_cached = self.field.is_cached
+        missing_instances = [i for i in instances if not is_cached(i)]
+        return prefetch_related_objects(missing_instances, self.field.name)
 
     def __set__(self, instance, value):
         ct = None

django/core/exceptions.py

@@ -132,6 +132,12 @@ class FieldError(Exception):
     pass
 
 
+class FieldFetchBlocked(FieldError):
+    """On-demand fetching of a model field blocked."""
+
+    pass
+
+
 NON_FIELD_ERRORS = "__all__"
 
 

django/db/models/__init__.py

@@ -36,6 +36,7 @@ from django.db.models.expressions import (
     WindowFrame,
     WindowFrameExclusion,
 )
+from django.db.models.fetch_modes import FETCH_ONE, FETCH_PEERS, RAISE
 from django.db.models.fields import *  # NOQA
 from django.db.models.fields import __all__ as fields_all
 from django.db.models.fields.composite import CompositePrimaryKey
@@ -105,6 +106,9 @@ __all__ += [
     "GeneratedField",
     "JSONField",
     "OrderWrt",
+    "FETCH_ONE",
+    "FETCH_PEERS",
+    "RAISE",
     "Lookup",
     "Transform",
     "Manager",

django/db/models/base.py

@@ -32,6 +32,7 @@ from django.db.models import NOT_PROVIDED, ExpressionWrapper, IntegerField, Max,
 from django.db.models.constants import LOOKUP_SEP
 from django.db.models.deletion import CASCADE, Collector
 from django.db.models.expressions import DatabaseDefault
+from django.db.models.fetch_modes import FETCH_ONE
 from django.db.models.fields.composite import CompositePrimaryKey
 from django.db.models.fields.related import (
     ForeignObjectRel,
@@ -466,6 +467,14 @@ class ModelStateFieldsCacheDescriptor:
         return res
 
 
+class ModelStateFetchModeDescriptor:
+    def __get__(self, instance, cls=None):
+        if instance is None:
+            return self
+        res = instance.fetch_mode = FETCH_ONE
+        return res
+
+
 class ModelState:
     """Store model instance state."""
 
@@ -476,6 +485,14 @@ class ModelState:
     # on the actual save.
     adding = True
     fields_cache = ModelStateFieldsCacheDescriptor()
+    fetch_mode = ModelStateFetchModeDescriptor()
+    peers = ()
+
+    def __getstate__(self):
+        state = self.__dict__.copy()
+        # Weak references can't be pickled.
+        state.pop("peers", None)
+        return state
 
 
 class Model(AltersData, metaclass=ModelBase):
@@ -595,7 +612,7 @@ class Model(AltersData, metaclass=ModelBase):
         post_init.send(sender=cls, instance=self)
 
     @classmethod
-    def from_db(cls, db, field_names, values):
+    def from_db(cls, db, field_names, values, *, fetch_mode=None):
         if len(values) != len(cls._meta.concrete_fields):
             values_iter = iter(values)
             values = [
@@ -605,6 +622,8 @@ class Model(AltersData, metaclass=ModelBase):
         new = cls(*values)
         new._state.adding = False
         new._state.db = db
+        if fetch_mode is not None:
+            new._state.fetch_mode = fetch_mode
         return new
 
     def __repr__(self):
@@ -714,8 +733,8 @@ class Model(AltersData, metaclass=ModelBase):
         should be an iterable of field attnames. If fields is None, then
         all non-deferred fields are reloaded.
 
-        When accessing deferred fields of an instance, the deferred loading
-        of the field will call this method.
+        When fetching deferred fields for a single instance (the FETCH_ONE
+        fetch mode), the deferred loading uses this method.
         """
         if fields is None:
             self._prefetched_objects_cache = {}

django/db/models/fetch_modes.py

@@ -0,0 +1,52 @@
+from django.core.exceptions import FieldFetchBlocked
+
+
+class FetchMode:
+    __slots__ = ()
+
+    track_peers = False
+
+    def fetch(self, fetcher, instance):
+        raise NotImplementedError("Subclasses must implement this method.")
+
+
+class FetchOne(FetchMode):
+    __slots__ = ()
+
+    def fetch(self, fetcher, instance):
+        fetcher.fetch_one(instance)
+
+
+FETCH_ONE = FetchOne()
+
+
+class FetchPeers(FetchMode):
+    __slots__ = ()
+
+    track_peers = True
+
+    def fetch(self, fetcher, instance):
+        instances = [
+            peer
+            for peer_weakref in instance._state.peers
+            if (peer := peer_weakref()) is not None
+        ]
+        if len(instances) > 1:
+            fetcher.fetch_many(instances)
+        else:
+            fetcher.fetch_one(instance)
+
+
+FETCH_PEERS = FetchPeers()
+
+
+class Raise(FetchMode):
+    __slots__ = ()
+
+    def fetch(self, fetcher, instance):
+        klass = instance.__class__.__qualname__
+        field_name = fetcher.field.name
+        raise FieldFetchBlocked(f"Fetching of {klass}.{field_name} blocked.") from None
+
+
+RAISE = Raise()

django/db/models/fields/related_descriptors.py

@@ -78,7 +78,7 @@ from django.db.models.expressions import ColPairs
 from django.db.models.fields.tuple_lookups import TupleIn
 from django.db.models.functions import RowNumber
 from django.db.models.lookups import GreaterThan, LessThanOrEqual
-from django.db.models.query import QuerySet
+from django.db.models.query import QuerySet, prefetch_related_objects
 from django.db.models.query_utils import DeferredAttribute
 from django.db.models.utils import AltersData, resolve_callables
 from django.utils.functional import cached_property
@@ -254,13 +254,9 @@ class ForwardManyToOneDescriptor:
                             break
 
             if rel_obj is None and has_value:
-                rel_obj = self.get_object(instance)
-                remote_field = self.field.remote_field
-                # If this is a one-to-one relation, set the reverse accessor
-                # cache on the related object to the current instance to avoid
-                # an extra SQL query if it's accessed later on.
-                if not remote_field.multiple:
-                    remote_field.set_cached_value(rel_obj, instance)
+                instance._state.fetch_mode.fetch(self, instance)
+                return self.field.get_cached_value(instance)
+
             self.field.set_cached_value(instance, rel_obj)
 
         if rel_obj is None and not self.field.null:
@@ -270,6 +266,21 @@ class ForwardManyToOneDescriptor:
         else:
             return rel_obj
 
+    def fetch_one(self, instance):
+        rel_obj = self.get_object(instance)
+        self.field.set_cached_value(instance, rel_obj)
+        # If this is a one-to-one relation, set the reverse accessor cache on
+        # the related object to the current instance to avoid an extra SQL
+        # query if it's accessed later on.
+        remote_field = self.field.remote_field
+        if not remote_field.multiple:
+            remote_field.set_cached_value(rel_obj, instance)
+
+    def fetch_many(self, instances):
+        is_cached = self.is_cached
+        missing_instances = [i for i in instances if not is_cached(i)]
+        prefetch_related_objects(missing_instances, self.field.name)
+
     def __set__(self, instance, value):
         """
         Set the related instance through the forward relation.
@@ -504,16 +515,8 @@ class ReverseOneToOneDescriptor:
             if not instance._is_pk_set():
                 rel_obj = None
             else:
-                filter_args = self.related.field.get_forward_related_filter(instance)
-                try:
-                    rel_obj = self.get_queryset(instance=instance).get(**filter_args)
-                except self.related.related_model.DoesNotExist:
-                    rel_obj = None
-                else:
-                    # Set the forward accessor cache on the related object to
-                    # the current instance to avoid an extra SQL query if it's
-                    # accessed later on.
-                    self.related.field.set_cached_value(rel_obj, instance)
+                instance._state.fetch_mode.fetch(self, instance)
+                rel_obj = self.related.get_cached_value(instance)
             self.related.set_cached_value(instance, rel_obj)
 
         if rel_obj is None:
@@ -524,6 +527,34 @@ class ReverseOneToOneDescriptor:
         else:
             return rel_obj
 
+    @property
+    def field(self):
+        """
+        Add compatibility with the fetcher protocol. While self.related is not
+        a field but a OneToOneRel, it quacks enough like a field to work.
+        """
+        return self.related
+
+    def fetch_one(self, instance):
+        # Kept for backwards compatibility with overridden
+        # get_forward_related_filter()
+        filter_args = self.related.field.get_forward_related_filter(instance)
+        try:
+            rel_obj = self.get_queryset(instance=instance).get(**filter_args)
+        except self.related.related_model.DoesNotExist:
+            rel_obj = None
+        else:
+            self.related.field.set_cached_value(rel_obj, instance)
+        self.related.set_cached_value(instance, rel_obj)
+
+    def fetch_many(self, instances):
+        is_cached = self.is_cached
+        missing_instances = [i for i in instances if not is_cached(i)]
+        prefetch_related_objects(
+            missing_instances,
+            self.related.get_accessor_name(),
+        )
+
     def __set__(self, instance, value):
         """
         Set the related instance through the reverse relation.

django/db/models/query.py

@@ -8,6 +8,7 @@ import warnings
 from contextlib import nullcontext
 from functools import reduce
 from itertools import chain, islice
+from weakref import ref as weak_ref
 
 from asgiref.sync import sync_to_async
 
@@ -26,6 +27,7 @@ from django.db.models import AutoField, DateField, DateTimeField, Field, Max, sq
 from django.db.models.constants import LOOKUP_SEP, OnConflict
 from django.db.models.deletion import Collector
 from django.db.models.expressions import Case, DatabaseDefault, F, Value, When
+from django.db.models.fetch_modes import FETCH_ONE
 from django.db.models.functions import Cast, Trunc
 from django.db.models.query_utils import FilteredRelation, Q
 from django.db.models.sql.constants import GET_ITERATOR_CHUNK_SIZE, ROW_COUNT
@@ -122,10 +124,18 @@ class ModelIterable(BaseIterable):
             )
             for field, related_objs in queryset._known_related_objects.items()
         ]
+        fetch_mode = queryset._fetch_mode
+        peers = []
         for row in compiler.results_iter(results):
             obj = model_cls.from_db(
-                db, init_list, row[model_fields_start:model_fields_end]
+                db,
+                init_list,
+                row[model_fields_start:model_fields_end],
+                fetch_mode=fetch_mode,
             )
+            if fetch_mode.track_peers:
+                peers.append(weak_ref(obj))
+                obj._state.peers = peers
             for rel_populator in related_populators:
                 rel_populator.populate(row, obj)
             if annotation_col_map:
@@ -183,10 +193,17 @@ class RawModelIterable(BaseIterable):
                 query_iterator = compiler.composite_fields_to_tuples(
                     query_iterator, cols
                 )
+            fetch_mode = self.queryset._fetch_mode
+            peers = []
             for values in query_iterator:
                 # Associate fields to values
                 model_init_values = [values[pos] for pos in model_init_pos]
-                instance = model_cls.from_db(db, model_init_names, model_init_values)
+                instance = model_cls.from_db(
+                    db, model_init_names, model_init_values, fetch_mode=fetch_mode
+                )
+                if fetch_mode.track_peers:
+                    peers.append(weak_ref(instance))
+                    instance._state.peers = peers
                 if annotation_fields:
                     for column, pos in annotation_fields:
                         setattr(instance, column, values[pos])
@@ -293,6 +310,7 @@ class QuerySet(AltersData):
         self._prefetch_done = False
         self._known_related_objects = {}  # {rel_field: {pk: rel_obj}}
         self._iterable_class = ModelIterable
+        self._fetch_mode = FETCH_ONE
         self._fields = None
         self._defer_next_filter = False
         self._deferred_filter = None
@@ -1442,6 +1460,7 @@ class QuerySet(AltersData):
             params=params,
             translations=translations,
             using=using,
+            fetch_mode=self._fetch_mode,
         )
         qs._prefetch_related_lookups = self._prefetch_related_lookups[:]
         return qs
@@ -1913,6 +1932,12 @@ class QuerySet(AltersData):
         clone._db = alias
         return clone
 
+    def fetch_mode(self, fetch_mode):
+        """Set the fetch mode for the QuerySet."""
+        clone = self._chain()
+        clone._fetch_mode = fetch_mode
+        return clone
+
     ###################################
     # PUBLIC INTROSPECTION ATTRIBUTES #
     ###################################
@@ -2051,6 +2076,7 @@ class QuerySet(AltersData):
         c._prefetch_related_lookups = self._prefetch_related_lookups[:]
         c._known_related_objects = self._known_related_objects
         c._iterable_class = self._iterable_class
+        c._fetch_mode = self._fetch_mode
         c._fields = self._fields
         return c
 
@@ -2186,6 +2212,7 @@ class RawQuerySet:
         translations=None,
         using=None,
         hints=None,
+        fetch_mode=FETCH_ONE,
     ):
         self.raw_query = raw_query
         self.model = model
@@ -2197,6 +2224,7 @@ class RawQuerySet:
         self._result_cache = None
         self._prefetch_related_lookups = ()
         self._prefetch_done = False
+        self._fetch_mode = fetch_mode
 
     def resolve_model_init_order(self):
         """Resolve the init field names and value positions."""
@@ -2295,6 +2323,7 @@ class RawQuerySet:
             params=self.params,
             translations=self.translations,
             using=alias,
+            fetch_mode=self._fetch_mode,
         )
 
     @cached_property

django/db/models/query_utils.py

@@ -264,7 +264,8 @@ class DeferredAttribute:
                         f"Cannot retrieve deferred field {field_name!r} "
                         "from an unsaved model."
                     )
-                instance.refresh_from_db(fields=[field_name])
+
+                instance._state.fetch_mode.fetch(self, instance)
             else:
                 data[field_name] = val
         return data[field_name]
@@ -281,6 +282,20 @@ class DeferredAttribute:
             return getattr(instance, link_field.attname)
         return None
 
+    def fetch_one(self, instance):
+        instance.refresh_from_db(fields=[self.field.attname])
+
+    def fetch_many(self, instances):
+        attname = self.field.attname
+        db = instances[0]._state.db
+        value_by_pk = (
+            self.field.model._base_manager.using(db)
+            .values_list(attname)
+            .in_bulk({i.pk for i in instances})
+        )
+        for instance in instances:
+            setattr(instance, attname, value_by_pk[instance.pk])
+
 
 class class_or_instance_method:
     """

docs/ref/exceptions.txt

@@ -165,6 +165,16 @@ Django core exception classes are defined in ``django.core.exceptions``.
     - A field name is invalid
     - A query contains invalid order_by arguments
 
+``FieldFetchBlocked``
+---------------------
+
+.. versionadded:: 6.1
+
+.. exception:: FieldFetchBlocked
+
+    Raised when a field would be fetched on-demand and the
+    :attr:`~django.db.models.RAISE` fetch mode is active.
+
 ``ValidationError``
 -------------------
 

docs/ref/models/instances.txt

@@ -180,10 +180,10 @@ update, you could write a test similar to this::
         obj.refresh_from_db()
         self.assertEqual(obj.val, 2)
 
-Note that when deferred fields are accessed, the loading of the deferred
-field's value happens through this method. Thus it is possible to customize
-the way deferred loading happens. The example below shows how one can reload
-all of the instance's fields when a deferred field is reloaded::
+When a deferred field is loaded on-demand for a single model instance, the
+loading happens through this method. Thus it is possible to customize the way
+this loading happens. The example below shows how one can reload all of the
+instance's fields when a deferred field is loaded on-demand::
 
     class ExampleModel(models.Model):
         def refresh_from_db(self, using=None, fields=None, **kwargs):

docs/ref/models/querysets.txt

@@ -1022,15 +1022,38 @@ Uses SQL's ``EXCEPT`` operator to keep only elements present in the
 
 See :meth:`union` for some restrictions.
 
+``fetch_mode()``
+~~~~~~~~~~~~~~~~
+
+.. versionadded:: 6.1
+
+.. method:: fetch_mode(mode)
+
+Returns a ``QuerySet`` that sets the given fetch mode for all model instances
+created by this ``QuerySet``. The fetch mode controls on-demand loading of
+fields when they are accessed, such as for foreign keys and deferred fields.
+For example, to use the :attr:`~django.db.models.FETCH_PEERS` mode to
+batch-load all related objects on first access:
+
+.. code-block:: python
+
+    from django.db import models
+
+    books = Book.objects.fetch_mode(models.FETCH_PEERS)
+
+See more in the :doc:`fetch mode topic guide </topics/db/fetch-modes>`.
+
 ``select_related()``
 ~~~~~~~~~~~~~~~~~~~~
 
 .. method:: select_related(*fields)
 
-Returns a ``QuerySet`` that will "follow" foreign-key relationships, selecting
-additional related-object data when it executes its query. This is a
-performance booster which results in a single more complex query but means
-later use of foreign-key relationships won't require database queries.
+Returns a ``QuerySet`` that will join in the named foreign-key relationships,
+selecting additional related objects when it executes its query. This method
+can be a performance booster, fetching data ahead of time rather than
+triggering on-demand loading through the model instances'
+:doc:`fetch mode </topics/db/fetch-modes>`, at the cost of a more complex
+initial query.
 
 The following examples illustrate the difference between plain lookups and
 ``select_related()`` lookups. Here's standard lookup::
@@ -1050,20 +1073,8 @@ And here's ``select_related`` lookup::
     # in the previous query.
     b = e.blog
 
-You can use ``select_related()`` with any queryset of objects::
-
-    from django.utils import timezone
-
-    # Find all the blogs with entries scheduled to be published in the future.
-    blogs = set()
-
-    for e in Entry.objects.filter(pub_date__gt=timezone.now()).select_related("blog"):
-        # Without select_related(), this would make a database query for each
-        # loop iteration in order to fetch the related blog for each entry.
-        blogs.add(e.blog)
-
-The order of ``filter()`` and ``select_related()`` chaining isn't important.
-These querysets are equivalent::
+You can use ``select_related()`` with any queryset. The order of chaining with
+other methods isn't important. For example, these querysets are equivalent::
 
     Entry.objects.filter(pub_date__gt=timezone.now()).select_related("blog")
     Entry.objects.select_related("blog").filter(pub_date__gt=timezone.now())
@@ -1141,12 +1152,15 @@ that is that ``select_related('foo', 'bar')`` is equivalent to
 
 .. method:: prefetch_related(*lookups)
 
-Returns a ``QuerySet`` that will automatically retrieve, in a single batch,
-related objects for each of the specified lookups.
+Returns a ``QuerySet`` that will automatically retrieve the given lookups, each
+in one extra batch query. Prefetching is a way to optimize database access
+when you know you'll be accessing related objects later, so you can avoid
+triggering the on-demand loading behavior of the model instances'
+:doc:`fetch mode </topics/db/fetch-modes>`.
 
-This has a similar purpose to ``select_related``, in that both are designed to
-stop the deluge of database queries that is caused by accessing related
-objects, but the strategy is quite different.
+This method has a similar purpose to :meth:`select_related`, in that both are
+designed to eagerly fetch related objects. However, they work in different
+ways.
 
 ``select_related`` works by creating an SQL join and including the fields of
 the related object in the ``SELECT`` statement. For this reason,

docs/releases/6.1.txt

@@ -26,6 +26,51 @@ only officially support, the latest release of each series.
 What's new in Django 6.1
 ========================
 
+Model field fetch modes
+-----------------------
+
+The on-demand fetching behavior of model fields is now configurable with
+:doc:`fetch modes </topics/db/fetch-modes>`. These modes allow you to control
+how Django fetches data from the database when an unfetched field is accessed.
+
+Django provides three fetch modes:
+
+1. ``FETCH_ONE``, the default, fetches the missing field for the current
+   instance only. This mode represents Django's existing behavior.
+
+2. ``FETCH_PEERS`` fetches a missing field for all instances that came from
+   the same :class:`~django.db.models.query.QuerySet`.
+
+   This mode works like an on-demand ``prefetch_related()``. It can reduce most
+   cases of the "N+1 queries problem" to two queries without any work to
+   maintain a list of fields to prefetch.
+
+3. ``RAISE`` raises a :exc:`~django.core.exceptions.FieldFetchBlocked`
+   exception.
+
+   This mode can prevent unintentional queries in performance-critical
+   sections of code.
+
+Use the new method :meth:`.QuerySet.fetch_mode` to set the fetch mode for model
+instances fetched by the ``QuerySet``:
+
+.. code-block:: python
+
+    from django.db import models
+
+    books = Book.objects.fetch_mode(models.FETCH_PEERS)
+    for book in books:
+        print(book.author.name)
+
+Despite the loop accessing the ``author`` foreign key on each instance, the
+``FETCH_PEERS`` fetch mode will make the above example perform only two
+queries:
+
+1. Fetch all books.
+2. Fetch associated authors.
+
+See :doc:`fetch modes </topics/db/fetch-modes>` for more details.
+
 Minor features
 --------------
 

docs/spelling_wordlist

@@ -535,6 +535,7 @@ unencrypted
 unescape
 unescaped
 unevaluated
+unfetched
 unglamorous
 ungrouped
 unhandled

docs/topics/db/fetch-modes.txt

@@ -0,0 +1,138 @@
+===========
+Fetch modes
+===========
+
+.. versionadded:: 6.1
+
+.. module:: django.db.models.fetch_modes
+
+.. currentmodule:: django.db.models
+
+When accessing model fields that were not loaded as part of the original query,
+Django will fetch that field's data from the database. You can customize the
+behavior of this fetching with a **fetch mode**, making it more efficient or
+even blocking it.
+
+Use :meth:`.QuerySet.fetch_mode` to set the fetch mode for model
+instances fetched by a ``QuerySet``:
+
+.. code-block:: python
+
+    from django.db import models
+
+    books = Book.objects.fetch_mode(models.FETCH_PEERS)
+
+Fetch modes apply to:
+
+* :class:`~django.db.models.ForeignKey` fields
+* :class:`~django.db.models.OneToOneField` fields and their reverse accessors
+* Fields deferred with :meth:`.QuerySet.defer` or :meth:`.QuerySet.only`
+* :ref:`generic-relations`
+
+Available modes
+===============
+
+.. admonition:: Referencing fetch modes
+
+    Fetch modes are defined in ``django.db.models.fetch_modes``, but for
+    convenience they're imported into :mod:`django.db.models`. The standard
+    convention is to use ``from django.db import models`` and refer to the
+    fetch modes as ``models.<mode>``.
+
+Django provides three fetch modes. We'll explain them below using these models:
+
+.. code-block:: python
+
+    from django.db import models
+
+
+    class Author(models.Model): ...
+
+
+    class Book(models.Model):
+        author = models.ForeignKey(Author, on_delete=models.CASCADE)
+        ...
+
+…and this loop:
+
+.. code-block:: python
+
+    for book in books:
+        print(book.author.name)
+
+…where ``books`` is a ``QuerySet`` of ``Book`` instances using some fetch mode.
+
+.. attribute:: FETCH_ONE
+
+Fetches the missing field for the current instance only. This is the default
+mode.
+
+Using ``FETCH_ONE`` for the above example would use:
+
+* 1 query to fetch ``books``
+* N queries, where N is the number of books, to fetch the missing ``author``
+  field
+
+…for a total of 1+N queries. This query pattern is known as the "N+1 queries
+problem" because it often leads to performance issues when N is large.
+
+.. attribute:: FETCH_PEERS
+
+Fetches the missing field for the current instance and its "peers"—instances
+that came from the same initial ``QuerySet``. The behavior of this mode is
+based on the assumption that if you need a field for one instance, you probably
+need it for all instances in the same batch, since you'll likely process them
+all identically.
+
+Using ``FETCH_PEERS`` for the above example would use:
+
+* 1 query to fetch ``books``
+* 1 query to fetch all missing ``author`` fields for the batch of books
+
+…for a total of 2 queries. The batch query makes this mode a lot more efficient
+than ``FETCH_ONE`` and is similar to an on-demand call to
+:meth:`.QuerySet.prefetch_related` or
+:func:`~django.db.models.prefetch_related_objects`. Using ``FETCH_PEERS`` can
+reduce most cases of the "N+1 queries problem" to two queries without
+much effort.
+
+The "peer" instances are tracked in a list of weak references, to avoid
+memory leaks where some peer instances are discarded.
+
+.. attribute:: RAISE
+
+Raises a :exc:`~django.core.exceptions.FieldFetchBlocked` exception.
+
+Using ``RAISE`` for the above example would raise an exception at the access of
+``book.author`` access, like:
+
+.. code-block:: python
+
+    FieldFetchBlocked("Fetching of Primary.value blocked.")
+
+This mode can prevent unintentional queries in performance-critical
+sections of code.
+
+.. _fetch-modes-custom-manager:
+
+Make a fetch mode the default for a model class
+===============================================
+
+Set the default fetch mode for a model class with a
+:ref:`custom manager <custom-managers>` that overrides ``get_queryset()``:
+
+.. code-block:: python
+
+    from django.db import models
+
+
+    class BookManager(models.Manager):
+        def get_queryset(self):
+            return super().get_queryset().fetch_mode(models.FETCH_PEERS)
+
+
+    class Book(models.Model):
+        title = models.TextField()
+        author = models.ForeignKey("Author", on_delete=models.CASCADE)
+
+        objects = BookManager()

docs/topics/db/index.txt

@@ -13,6 +13,7 @@ Generally, each model maps to a single database table.
 
    models
    queries
+   fetch-modes
    aggregation
    search
    managers

docs/topics/db/optimization.txt

@@ -196,28 +196,46 @@ thousands of records are returned. The penalty will be compounded if the
 database lives on a separate server, where network overhead and latency also
 play a factor.
 
-Retrieve everything at once if you know you will need it
-========================================================
+Retrieve related objects efficiently
+====================================
 
-Hitting the database multiple times for different parts of a single 'set' of
-data that you will need all parts of is, in general, less efficient than
-retrieving it all in one query. This is particularly important if you have a
-query that is executed in a loop, and could therefore end up doing many
-database queries, when only one was needed. So:
+Generally, accessing the database multiple times to retrieve different parts
+of a single "set" of data is less efficient than retrieving it all in one
+query. This is particularly important if you have a query that is executed in a
+loop, and could therefore end up doing many database queries, when only one
+is needed. Below are some techniques to combine queries for efficiency.
+
+Use the ``FETCH_PEERS`` fetch mode
+----------------------------------
+
+Use the :attr:`~django.db.models.FETCH_PEERS` fetch mode to make on-demand
+field access more efficient with bulk-fetching. Enable all it for all usage of
+your models :ref:`with a custom manager <fetch-modes-custom-manager>`.
+
+Using this fetch mode is easier than declaring fields to fetch with
+:meth:`~django.db.models.query.QuerySet.select_related` or
+:meth:`~django.db.models.query.QuerySet.prefetch_related`, especially when it's
+hard to predict which fields will be accessed.
 
 Use ``QuerySet.select_related()`` and ``prefetch_related()``
 ------------------------------------------------------------
 
-Understand :meth:`~django.db.models.query.QuerySet.select_related` and
-:meth:`~django.db.models.query.QuerySet.prefetch_related` thoroughly, and use
-them:
+When the :attr:`~django.db.models.FETCH_PEERS` fetch mode is not appropriate or
+efficient enough, use :meth:`~django.db.models.query.QuerySet.select_related`
+and :meth:`~django.db.models.query.QuerySet.prefetch_related`. Understand their
+documentation thoroughly and apply them where needed.
 
-* in :doc:`managers and default managers </topics/db/managers>` where
-  appropriate. Be aware when your manager is and is not used; sometimes this is
-  tricky so don't make assumptions.
+It may be useful to apply these methods in :doc:`managers and default managers
+</topics/db/managers>`. Be aware when your manager is and is not used;
+sometimes this is tricky so don't make assumptions.
 
-* in view code or other layers, possibly making use of
-  :func:`~django.db.models.prefetch_related_objects` where needed.
+Use ``prefetch_related_objects()``
+----------------------------------
+
+Where :attr:`~django.db.models.query.QuerySet.prefetch_related` would be useful
+after the queryset has been evaluated, use
+:func:`~django.db.models.prefetch_related_objects` to execute an extra
+prefetch.
 
 Don't retrieve things you don't need
 ====================================

docs/topics/db/queries.txt

@@ -1702,6 +1702,12 @@ the link from the related model to the model that defines the relationship.
 For example, a ``Blog`` object ``b`` has a manager that returns all related
 ``Entry`` objects in the ``entry_set`` attribute: ``b.entry_set.all()``.
 
+These accessors may be prefetched by the ``QuerySet`` methods
+:meth:`~django.db.models.query.QuerySet.select_related` or
+:meth:`~django.db.models.query.QuerySet.prefetch_related`. If not prefetched,
+access will trigger an on-demand fetch through the model's
+:doc:`fetch mode </topics/db/fetch-modes>`.
+
 All examples in this section use the sample ``Blog``, ``Author`` and ``Entry``
 models defined at the top of this page.
 

tests/basic/tests.py

@@ -807,6 +807,7 @@ class ManagerTest(SimpleTestCase):
         "alatest",
         "aupdate",
         "aupdate_or_create",
+        "fetch_mode",
     ]
 
     def test_manager_methods(self):

tests/defer/tests.py

@@ -1,4 +1,5 @@
-from django.core.exceptions import FieldDoesNotExist, FieldError
+from django.core.exceptions import FieldDoesNotExist, FieldError, FieldFetchBlocked
+from django.db.models import FETCH_PEERS, RAISE
 from django.test import SimpleTestCase, TestCase
 
 from .models import (
@@ -29,6 +30,7 @@ class DeferTests(AssertionMixin, TestCase):
     def setUpTestData(cls):
         cls.s1 = Secondary.objects.create(first="x1", second="y1")
         cls.p1 = Primary.objects.create(name="p1", value="xx", related=cls.s1)
+        cls.p2 = Primary.objects.create(name="p2", value="yy", related=cls.s1)
 
     def test_defer(self):
         qs = Primary.objects.all()
@@ -141,7 +143,6 @@ class DeferTests(AssertionMixin, TestCase):
     def test_saving_object_with_deferred_field(self):
         # Saving models with deferred fields is possible (but inefficient,
         # since every field has to be retrieved first).
-        Primary.objects.create(name="p2", value="xy", related=self.s1)
         obj = Primary.objects.defer("value").get(name="p2")
         obj.name = "a new name"
         obj.save()
@@ -181,10 +182,71 @@ class DeferTests(AssertionMixin, TestCase):
         self.assertEqual(obj.name, "adonis")
 
     def test_defer_fk_attname(self):
-        primary = Primary.objects.defer("related_id").get()
+        primary = Primary.objects.defer("related_id").get(name="p1")
         with self.assertNumQueries(1):
             self.assertEqual(primary.related_id, self.p1.related_id)
 
+    def test_only_fetch_mode_fetch_peers(self):
+        p1, p2 = Primary.objects.fetch_mode(FETCH_PEERS).only("name")
+        with self.assertNumQueries(1):
+            p1.value
+        with self.assertNumQueries(0):
+            p2.value
+
+    def test_only_fetch_mode_fetch_peers_single(self):
+        p1 = Primary.objects.fetch_mode(FETCH_PEERS).only("name").get(name="p1")
+        with self.assertNumQueries(1):
+            p1.value
+
+    def test_defer_fetch_mode_fetch_peers(self):
+        p1, p2 = Primary.objects.fetch_mode(FETCH_PEERS).defer("value")
+        with self.assertNumQueries(1):
+            p1.value
+        with self.assertNumQueries(0):
+            p2.value
+
+    def test_defer_fetch_mode_fetch_peers_single(self):
+        p1 = Primary.objects.fetch_mode(FETCH_PEERS).defer("value").get(name="p1")
+        with self.assertNumQueries(1):
+            p1.value
+
+    def test_only_fetch_mode_raise(self):
+        p1 = Primary.objects.fetch_mode(RAISE).only("name").get(name="p1")
+        msg = "Fetching of Primary.value blocked."
+        with self.assertRaisesMessage(FieldFetchBlocked, msg) as cm:
+            p1.value
+        self.assertIsNone(cm.exception.__cause__)
+        self.assertTrue(cm.exception.__suppress_context__)
+
+    def test_defer_fetch_mode_raise(self):
+        p1 = Primary.objects.fetch_mode(RAISE).defer("value").get(name="p1")
+        msg = "Fetching of Primary.value blocked."
+        with self.assertRaisesMessage(FieldFetchBlocked, msg) as cm:
+            p1.value
+        self.assertIsNone(cm.exception.__cause__)
+        self.assertTrue(cm.exception.__suppress_context__)
+
+
+class DeferOtherDatabaseTests(TestCase):
+    databases = {"other"}
+
+    @classmethod
+    def setUpTestData(cls):
+        cls.s1 = Secondary.objects.using("other").create(first="x1", second="y1")
+        cls.p1 = Primary.objects.using("other").create(
+            name="p1", value="xx", related=cls.s1
+        )
+        cls.p2 = Primary.objects.using("other").create(
+            name="p2", value="yy", related=cls.s1
+        )
+
+    def test_defer_fetch_mode_fetch_peers(self):
+        p1, p2 = Primary.objects.using("other").fetch_mode(FETCH_PEERS).defer("value")
+        with self.assertNumQueries(1, using="other"):
+            p1.value
+        with self.assertNumQueries(0, using="other"):
+            p2.value
+
 
 class BigChildDeferTests(AssertionMixin, TestCase):
     @classmethod

tests/generic_relations/tests.py

@@ -1,7 +1,8 @@
 from django.contrib.contenttypes.models import ContentType
 from django.contrib.contenttypes.prefetch import GenericPrefetch
-from django.core.exceptions import FieldError
+from django.core.exceptions import FieldError, FieldFetchBlocked
 from django.db.models import Q, prefetch_related_objects
+from django.db.models.fetch_modes import FETCH_PEERS, RAISE
 from django.test import SimpleTestCase, TestCase, skipUnlessDBFeature
 
 from .models import (
@@ -780,6 +781,46 @@ class GenericRelationsTests(TestCase):
                 self.platypus.latin_name,
             )
 
+    def test_fetch_mode_fetch_peers(self):
+        TaggedItem.objects.bulk_create(
+            [
+                TaggedItem(tag="lion", content_object=self.lion),
+                TaggedItem(tag="platypus", content_object=self.platypus),
+                TaggedItem(tag="quartz", content_object=self.quartz),
+            ]
+        )
+        # Peers fetching should fetch all related peers GFKs at once which is
+        # one query per content type.
+        with self.assertNumQueries(1):
+            quartz_tag, platypus_tag, lion_tag = TaggedItem.objects.fetch_mode(
+                FETCH_PEERS
+            ).order_by("-pk")[:3]
+        with self.assertNumQueries(2):
+            self.assertEqual(lion_tag.content_object, self.lion)
+        with self.assertNumQueries(0):
+            self.assertEqual(platypus_tag.content_object, self.platypus)
+            self.assertEqual(quartz_tag.content_object, self.quartz)
+        # It should ignore already cached instances though.
+        with self.assertNumQueries(1):
+            quartz_tag, platypus_tag, lion_tag = TaggedItem.objects.fetch_mode(
+                FETCH_PEERS
+            ).order_by("-pk")[:3]
+        with self.assertNumQueries(2):
+            self.assertEqual(quartz_tag.content_object, self.quartz)
+            self.assertEqual(lion_tag.content_object, self.lion)
+        with self.assertNumQueries(0):
+            self.assertEqual(platypus_tag.content_object, self.platypus)
+            self.assertEqual(quartz_tag.content_object, self.quartz)
+
+    def test_fetch_mode_raise(self):
+        TaggedItem.objects.create(tag="lion", content_object=self.lion)
+        tag = TaggedItem.objects.fetch_mode(RAISE).get(tag="yellow")
+        msg = "Fetching of TaggedItem.content_object blocked."
+        with self.assertRaisesMessage(FieldFetchBlocked, msg) as cm:
+            tag.content_object
+        self.assertIsNone(cm.exception.__cause__)
+        self.assertTrue(cm.exception.__suppress_context__)
+
 
 class ProxyRelatedModelTest(TestCase):
     def test_default_behavior(self):

tests/many_to_one/tests.py

@@ -1,8 +1,13 @@
 import datetime
 from copy import deepcopy
 
-from django.core.exceptions import FieldError, MultipleObjectsReturned
+from django.core.exceptions import (
+    FieldError,
+    FieldFetchBlocked,
+    MultipleObjectsReturned,
+)
 from django.db import IntegrityError, models, transaction
+from django.db.models import FETCH_PEERS, RAISE
 from django.test import TestCase
 from django.utils.translation import gettext_lazy
 
@@ -916,3 +921,23 @@ class ManyToOneTests(TestCase):
                 instances=countries,
                 querysets=[City.objects.all(), City.objects.all()],
             )
+
+    def test_fetch_mode_fetch_peers_forward(self):
+        Article.objects.create(
+            headline="This is another test",
+            pub_date=datetime.date(2005, 7, 27),
+            reporter=self.r2,
+        )
+        a1, a2 = Article.objects.fetch_mode(FETCH_PEERS)
+        with self.assertNumQueries(1):
+            a1.reporter
+        with self.assertNumQueries(0):
+            a2.reporter
+
+    def test_fetch_mode_raise_forward(self):
+        a = Article.objects.fetch_mode(RAISE).get(pk=self.a.pk)
+        msg = "Fetching of Article.reporter blocked."
+        with self.assertRaisesMessage(FieldFetchBlocked, msg) as cm:
+            a.reporter
+        self.assertIsNone(cm.exception.__cause__)
+        self.assertTrue(cm.exception.__suppress_context__)

tests/one_to_one/tests.py

@@ -1,4 +1,6 @@
+from django.core.exceptions import FieldFetchBlocked
 from django.db import IntegrityError, connection, transaction
+from django.db.models import FETCH_PEERS, RAISE
 from django.test import TestCase
 
 from .models import (
@@ -619,3 +621,39 @@ class OneToOneTests(TestCase):
                 instances=places,
                 querysets=[Bar.objects.all(), Bar.objects.all()],
             )
+
+    def test_fetch_mode_fetch_peers_forward(self):
+        Restaurant.objects.create(
+            place=self.p2, serves_hot_dogs=True, serves_pizza=False
+        )
+        r1, r2 = Restaurant.objects.fetch_mode(FETCH_PEERS)
+        with self.assertNumQueries(1):
+            r1.place
+        with self.assertNumQueries(0):
+            r2.place
+
+    def test_fetch_mode_fetch_peers_reverse(self):
+        Restaurant.objects.create(
+            place=self.p2, serves_hot_dogs=True, serves_pizza=False
+        )
+        p1, p2 = Place.objects.fetch_mode(FETCH_PEERS)
+        with self.assertNumQueries(1):
+            p1.restaurant
+        with self.assertNumQueries(0):
+            p2.restaurant
+
+    def test_fetch_mode_raise_forward(self):
+        r = Restaurant.objects.fetch_mode(RAISE).get(pk=self.r1.pk)
+        msg = "Fetching of Restaurant.place blocked."
+        with self.assertRaisesMessage(FieldFetchBlocked, msg) as cm:
+            r.place
+        self.assertIsNone(cm.exception.__cause__)
+        self.assertTrue(cm.exception.__suppress_context__)
+
+    def test_fetch_mode_raise_reverse(self):
+        p = Place.objects.fetch_mode(RAISE).get(pk=self.p1.pk)
+        msg = "Fetching of Place.restaurant blocked."
+        with self.assertRaisesMessage(FieldFetchBlocked, msg) as cm:
+            p.restaurant
+        self.assertIsNone(cm.exception.__cause__)
+        self.assertTrue(cm.exception.__suppress_context__)

tests/prefetch_related/tests.py

@@ -4,6 +4,7 @@ from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import ObjectDoesNotExist
 from django.db import NotSupportedError, connection
 from django.db.models import F, Prefetch, QuerySet, prefetch_related_objects
+from django.db.models.fetch_modes import RAISE
 from django.db.models.query import get_prefetcher
 from django.db.models.sql import Query
 from django.test import (
@@ -107,6 +108,10 @@ class PrefetchRelatedTests(TestDataMixin, TestCase):
         normal_books = [a.first_book for a in Author.objects.all()]
         self.assertEqual(books, normal_books)
 
+    def test_fetch_mode_raise(self):
+        authors = list(Author.objects.fetch_mode(RAISE).prefetch_related("first_book"))
+        authors[0].first_book  # No exception, already loaded
+
     def test_foreignkey_reverse(self):
         with self.assertNumQueries(2):
             [

tests/raw_query/tests.py

@@ -1,7 +1,8 @@
 from datetime import date
 from decimal import Decimal
 
-from django.core.exceptions import FieldDoesNotExist
+from django.core.exceptions import FieldDoesNotExist, FieldFetchBlocked
+from django.db.models import FETCH_PEERS, RAISE
 from django.db.models.query import RawQuerySet
 from django.test import TestCase, skipUnlessDBFeature
 
@@ -158,6 +159,22 @@ class RawQueryTests(TestCase):
         books = Book.objects.all()
         self.assertSuccessfulRawQuery(Book, query, books)
 
+    def test_fk_fetch_mode_peers(self):
+        query = "SELECT * FROM raw_query_book"
+        books = list(Book.objects.fetch_mode(FETCH_PEERS).raw(query))
+        with self.assertNumQueries(1):
+            books[0].author
+            books[1].author
+
+    def test_fk_fetch_mode_raise(self):
+        query = "SELECT * FROM raw_query_book"
+        books = list(Book.objects.fetch_mode(RAISE).raw(query))
+        msg = "Fetching of Book.author blocked."
+        with self.assertRaisesMessage(FieldFetchBlocked, msg) as cm:
+            books[0].author
+        self.assertIsNone(cm.exception.__cause__)
+        self.assertTrue(cm.exception.__suppress_context__)
+
     def test_db_column_handler(self):
         """
         Test of a simple raw query against a model containing a field with
@@ -294,6 +311,23 @@ class RawQueryTests(TestCase):
         with self.assertRaisesMessage(FieldDoesNotExist, msg):
             list(Author.objects.raw(query))
 
+    def test_missing_fields_fetch_mode_peers(self):
+        query = "SELECT id, first_name, dob FROM raw_query_author"
+        authors = list(Author.objects.fetch_mode(FETCH_PEERS).raw(query))
+        with self.assertNumQueries(1):
+            authors[0].last_name
+            authors[1].last_name
+
+    def test_missing_fields_fetch_mode_raise(self):
+        query = "SELECT id, first_name, dob FROM raw_query_author"
+        authors = list(Author.objects.fetch_mode(RAISE).raw(query))
+        msg = "Fetching of Author.last_name blocked."
+        with self.assertRaisesMessage(FieldFetchBlocked, msg) as cm:
+            authors[0].last_name
+        self.assertIsNone(cm.exception.__cause__)
+        self.assertTrue(cm.exception.__suppress_context__)
+        self.assertTrue(cm.exception.__suppress_context__)
+
     def test_annotations(self):
         query = (
             "SELECT a.*, count(b.id) as book_count "