Deprecated depth kwarg on select_related.

This is the start of a deprecation path for the depth kwarg on
select_related. Removing this will allow us to update select_related so
it chains properly and have an API similar to prefetch_related.

Thanks to Marc Tamlyn for spearheading and initial patch.

refs #16855

django/db/models/query.py

@@ -5,6 +5,7 @@ The main QuerySet implementation. This provides the public API for the ORM.
 import copy
 import itertools
 import sys
+import warnings
 
 from django.core import exceptions
 from django.db import connections, router, transaction, IntegrityError
@@ -698,6 +699,9 @@ class QuerySet(object):
         If fields are specified, they must be ForeignKey fields and only those
         related objects are included in the selection.
         """
+        if 'depth' in kwargs:
+            warnings.warn('The "depth" keyword argument has been deprecated.\n'
+                    'Use related field names instead.', PendingDeprecationWarning)
         depth = kwargs.pop('depth', 0)
         if kwargs:
             raise TypeError('Unexpected keyword arguments to select_related: %s'

docs/internals/deprecation.txt

@@ -298,6 +298,9 @@ these changes.
 
 * The ``daily_cleanup.py`` script will be removed.
 
+* The ``depth`` keyword argument will be removed from
+  :meth:`~django.db.models.query.QuerySet.select_related`.
+
 2.0
 ---
 

docs/ref/models/querysets.txt

@@ -676,21 +676,12 @@ Note that, by default, ``select_related()`` does not follow foreign keys that
 have ``null=True``.
 
 Usually, using ``select_related()`` can vastly improve performance because your
-app can avoid many database calls. However, in situations with deeply nested
-sets of relationships ``select_related()`` can sometimes end up following "too
-many" relations, and can generate queries so large that they end up being slow.
+app can avoid many database calls. However, there are times you are only
+interested in specific related models, or have deeply nested sets of
+relationships, and in these cases ``select_related()`` can can be optimized by
+explicitly passing the related field names you are interested in. Only
+the specified relations will be followed.
 
-In these situations, you can use the ``depth`` argument to ``select_related()``
-to control how many "levels" of relations ``select_related()`` will actually
-follow::
-
-    b = Book.objects.select_related(depth=1).get(id=4)
-    p = b.author         # Doesn't hit the database.
-    c = p.hometown       # Requires a database call.
-
-Sometimes you only want to access specific models that are related to your root
-model, not all of the related models. In these cases, you can pass the related
-field names to ``select_related()`` and it will only follow those relations.
 You can even do this for models that are more than one relation away by
 separating the field names with double underscores, just as for filters. For
 example, if you have this model::
@@ -730,6 +721,17 @@ You can also refer to the reverse direction of a
 is defined. Instead of specifying the field name, use the :attr:`related_name
 <django.db.models.ForeignKey.related_name>` for the field on the related object.
 
+.. deprecated:: 1.5
+    The ``depth`` parameter to ``select_related()`` has been deprecated. You
+    should replace it with the use of the ``(*fields)`` listing specific
+    related fields instead as documented above.
+
+A depth limit of relationships to follow can also be specified::
+
+    b = Book.objects.select_related(depth=1).get(id=4)
+    p = b.author         # Doesn't hit the database.
+    c = p.hometown       # Requires a database call.
+
 A :class:`~django.db.models.OneToOneField` is not traversed in the reverse
 direction if you are performing a depth-based ``select_related()`` call.
 

docs/releases/1.5.txt

@@ -638,3 +638,10 @@ The :djadmin:`cleanup` management command has been deprecated and replaced by
 
 The undocumented ``daily_cleanup.py`` script has been deprecated. Use the
 :djadmin:`clearsessions` management command instead.
+
+``depth`` keyword argument in ``select_related``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The ``depth`` keyword argument in
+:meth:`~django.db.models.query.QuerySet.select_related` has been deprecated.
+You should use field names instead.