Fixed #26207 -- Replaced dynamic classes with non-data descriptors for deferred instance loading.

1 files changed, 24 insertions, 13 deletions

diff --git a/docs/ref/models/instances.txt b/docs/ref/models/instances.txt
index f1fc3e60e6..e63db64ae0 100644
--- a/docs/ref/models/instances.txt
+++ b/docs/ref/models/instances.txt
@@ -73,13 +73,12 @@ when loading from the database.
 The ``db`` argument contains the database alias for the database the model
 is loaded from, ``field_names`` contains the names of all loaded fields, and
 ``values`` contains the loaded values for each field in ``field_names``. The
-``field_names`` are in the same order as the ``values``, so it is possible to
-use ``cls(**(zip(field_names, values)))`` to instantiate the object. If all
-of the model's fields are present, then ``values`` are guaranteed to be in
-the order ``__init__()`` expects them. That is, the instance can be created
-by ``cls(*values)``. It is possible to check if all fields are present by
-consulting ``cls._deferred`` - if ``False``, then all fields have been loaded
-from the database.
+``field_names`` are in the same order as the ``values``. If all of the model's
+fields are present, then ``values`` are guaranteed to be in the order
+``__init__()`` expects them. That is, the instance can be created by
+``cls(*values)``. If any fields are deferred, they won't appear in
+``field_names``. In that case, assign a value of ``django.db.models.DEFERRED``
+to each of the missing fields.
 
 In addition to creating the new model, the ``from_db()`` method must set the
 ``adding`` and ``db`` flags in the new instance's ``_state`` attribute.
@@ -87,14 +86,20 @@ In addition to creating the new model, the ``from_db()`` method must set the
 Below is an example showing how to record the initial values of fields that
 are loaded from the database::
 
+    from django.db.models import DEFERRED
+
     @classmethod
     def from_db(cls, db, field_names, values):
-        # default implementation of from_db() (could be replaced
-        # with super())
-        if cls._deferred:
-            instance = cls(**zip(field_names, values))
-        else:
-            instance = cls(*values)
+        # Default implementation of from_db() (subject to change and could
+        # be replaced with super()).
+        if len(values) != len(cls._meta.concrete_fields):
+            values = list(values)
+            values.reverse()
+            values = [
+                values.pop() if f.attname in field_names else DEFERRED
+                for f in cls._meta.concrete_fields
+            ]
+        new = cls(*values)
         instance._state.adding = False
         instance._state.db = db
         # customization to store the original field values on the instance
@@ -114,6 +119,12 @@ The example above shows a full ``from_db()`` implementation to clarify how that
 is done. In this case it would of course be possible to just use ``super()`` call
 in the ``from_db()`` method.
 
+.. versionchanged:: 1.10
+
+    In older versions, you could check if all fields were loaded by consulting
+    ``cls._deferred``. This attribute is removed and
+    ``django.db.models.DEFERRED`` is new.
+
 Refreshing objects from database
 ================================