From f2452936e9a33ce6eb95b11bd89b34fc6d2991a0 Mon Sep 17 00:00:00 2001
From: Ryan Hiebert <ryan@ryanhiebert.com>
Date: Thu, 5 Nov 2015 13:07:40 -0600
Subject: [PATCH] Allow no queryset when get_queryset overridden

The user may wish to provide a dynamic queryset on a `RelatedField`
based on the `context`. The way to do that is to create a subclass of
`RelatedField` (or a child) and override the `get_queryset` method.
However, this is undocumented, and instantiating that field without a
`queryset` argument (because it's not needed) will raise an assertion
error.

Document `.get_queryset(self)` as an official part of the API of
`RelatedField`, and don't enforce the use of `queryset` when
`get_queryset` is overridden.
---
 docs/api-guide/relations.md | 2 ++
 rest_framework/relations.py | 8 ++++----
 tests/test_relations.py     | 8 ++++++++
 3 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/docs/api-guide/relations.md b/docs/api-guide/relations.md
index 51e5f78c4..d5a7bfc7f 100644
--- a/docs/api-guide/relations.md
+++ b/docs/api-guide/relations.md
@@ -330,6 +330,8 @@ To implement a custom relational field, you should override `RelatedField`, and
 
 If you want to implement a read-write relational field, you must also implement the `.to_internal_value(self, data)` method.
 
+To provide a dynamic queryset based on the `context`, you can also override `.get_queryset(self)` instead of specifying `.queryset` on the class or when initializing the field.
+
 ## Example
 
 For example, we could define a relational field to serialize a track to a custom string representation, using its ordering, title, and duration.
diff --git a/rest_framework/relations.py b/rest_framework/relations.py
index 40261f3f3..74e0f3e03 100644
--- a/rest_framework/relations.py
+++ b/rest_framework/relations.py
@@ -62,10 +62,6 @@ class RelatedField(Field):
         self.queryset = kwargs.pop('queryset', self.queryset)
         self.html_cutoff = kwargs.pop('html_cutoff', self.html_cutoff)
         self.html_cutoff_text = kwargs.pop('html_cutoff_text', self.html_cutoff_text)
-        assert self.queryset is not None or kwargs.get('read_only', None), (
-            'Relational field must provide a `queryset` argument, '
-            'or set read_only=`True`.'
-        )
         assert not (self.queryset is not None and kwargs.get('read_only', None)), (
             'Relational fields should not provide a `queryset` argument, '
             'when setting read_only=`True`.'
@@ -112,6 +108,10 @@ class RelatedField(Field):
 
     def get_queryset(self):
         queryset = self.queryset
+        assert queryset is not None, (
+            'Relational field must provide a `queryset` argument, '
+            'or set read_only=`True`.'
+        )
         if isinstance(queryset, (QuerySet, Manager)):
             # Ensure queryset is re-evaluated whenever used.
             # Note that actually a `Manager` class may also be used as the
diff --git a/tests/test_relations.py b/tests/test_relations.py
index fd37e63e3..9703c3b9d 100644
--- a/tests/test_relations.py
+++ b/tests/test_relations.py
@@ -176,6 +176,14 @@ class TestSlugRelatedField(APISimpleTestCase):
         representation = self.field.to_representation(self.instance)
         assert representation == self.instance.name
 
+    def test_no_queryset_init(self):
+        class NoQuerySetSlugRelatedField(serializers.SlugRelatedField):
+            def get_queryset(this):
+                return self.queryset
+
+        field = NoQuerySetSlugRelatedField(slug_field='name')
+        field.to_internal_value(self.instance.name)
+
 
 class TestManyRelatedField(APISimpleTestCase):
     def setUp(self):