Fix schema generation for PrimaryKeyRelatedField

By default all subclasses of RelatedField are output as string fields in
the schema, which works well for StringRelatedField, SlugRelatedField or
HyperlinkedRelatedField.

Handle the common case of a PrimaryKeyRelatedField pointing to an
AutoField.

rest_framework/schemas/inspectors.py

@@ -50,6 +50,14 @@ def field_to_schema(field):
             title=title,
             description=description
         )
+    elif isinstance(field, serializers.PrimaryKeyRelatedField):
+        schema_cls = coreschema.String
+        model = getattr(field.queryset, 'model', None)
+        if model is not None:
+            model_field = model._meta.pk
+            if isinstance(model_field, models.AutoField):
+                schema_cls = coreschema.Integer
+        return schema_cls(title=title, description=description)
     elif isinstance(field, serializers.RelatedField):
         return coreschema.String(title=title, description=description)
     elif isinstance(field, serializers.MultipleChoiceField):

tests/test_schemas.py

@@ -25,7 +25,7 @@ from rest_framework.utils import formatting
 from rest_framework.views import APIView
 from rest_framework.viewsets import GenericViewSet, ModelViewSet
 
-from .models import BasicModel
+from .models import BasicModel, ForeignKeySource
 
 factory = APIRequestFactory()
 
@@ -558,6 +558,51 @@ class TestSchemaGeneratorWithRestrictedViewSets(TestCase):
         assert schema == expected
 
 
+class ForeignKeySourceSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = ForeignKeySource
+        fields = ('id', 'name', 'target')
+
+
+class ForeignKeySourceView(generics.CreateAPIView):
+    queryset = ForeignKeySource.objects.all()
+    serializer_class = ForeignKeySourceSerializer
+
+
+@unittest.skipUnless(coreapi, 'coreapi is not installed')
+class TestSchemaGeneratorWithForeignKey(TestCase):
+    def setUp(self):
+        self.patterns = [
+            url(r'^example/?$', ForeignKeySourceView.as_view()),
+        ]
+
+    def test_schema_for_regular_views(self):
+        """
+        Ensure that AutoField foreign keys are output as Integer.
+        """
+        generator = SchemaGenerator(title='Example API', patterns=self.patterns)
+        schema = generator.get_schema()
+
+        expected = coreapi.Document(
+            url='',
+            title='Example API',
+            content={
+                'example': {
+                    'create': coreapi.Link(
+                        url='/example/',
+                        action='post',
+                        encoding='application/json',
+                        fields=[
+                            coreapi.Field('name', required=True, location='form', schema=coreschema.String(title='Name')),
+                            coreapi.Field('target', required=True, location='form', schema=coreschema.Integer(description='Target', title='Target')),
+                        ]
+                    )
+                }
+            }
+        )
+        assert schema == expected
+
+
 @unittest.skipUnless(coreapi, 'coreapi is not installed')
 class Test4605Regression(TestCase):
     def test_4605_regression(self):