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

@@ -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.

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

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):