Document the use of get_queryset for RelatedField

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

@@ -21,6 +21,15 @@
 from rest_framework.utils import html
 
 
+def method_overridden(method_name, klass, instance):
+    """
+    Determine if a method has been overridden.
+    """
+    method = getattr(klass, method_name)
+    default_method = getattr(method, '__func__', method)  # Python 3 compat
+    return default_method is not getattr(instance, method_name).__func__
+
+
 class Hyperlink(six.text_type):
     """
     A string like object that additionally has an associated name.
@@ -62,10 +71,12 @@ def __init__(self, **kwargs):
         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`.'
-        )
+
+        if not method_overridden('get_queryset', RelatedField, self):
+            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`.'

tests/test_relations.py

@@ -176,6 +176,14 @@ def test_representation(self):
         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):