Add support for models with custom primary key

- serializer can treat primary key field as read-only during
  update, but accept it as writable during create when it is
  not auto-generated

rest_framework/serializers.py

@@ -994,7 +994,7 @@ class ModelSerializer(Serializer):
 
         # Determine any extra field arguments and hidden fields that
         # should be included
-        extra_kwargs = self.get_extra_kwargs()
+        extra_kwargs = self.get_extra_kwargs(info)
         extra_kwargs, hidden_fields = self.get_uniqueness_extra_kwargs(
             field_names, declared_fields, extra_kwargs
         )
@@ -1289,7 +1289,7 @@ class ModelSerializer(Serializer):
 
     # Methods for determining additional keyword arguments to apply...
 
-    def get_extra_kwargs(self):
+    def get_extra_kwargs(self, model_info):
         """
         Return a dictionary mapping field names to a dictionary of
         additional keyword arguments.
@@ -1317,6 +1317,8 @@ class ModelSerializer(Serializer):
                 (self.__class__.__module__, self.__class__.__name__)
             )
 
+        if not model_info.pk.auto_created and self.instance:
+            extra_kwargs.setdefault(model_info.pk.name, {})['read_only'] = True
         return extra_kwargs
 
     def get_uniqueness_extra_kwargs(self, field_names, declared_fields, extra_kwargs):

tests/test_model_serializer.py

@@ -42,6 +42,10 @@ class OneFieldModel(models.Model):
     char_field = models.CharField(max_length=100)
 
 
+class CustomPrimaryKeyModel(models.Model):
+    char_field = models.CharField(primary_key=True, max_length=100)
+
+
 class RegularFieldsModel(models.Model):
     """
     A model class for testing regular flat fields.
@@ -153,6 +157,34 @@ class TestModelSerializer(TestCase):
         assert str(excinfo.exception).startswith(msginitial)
 
 
+class TestCustomPrimaryKeyModelSerializer(TestCase):
+    class TestSerializer(serializers.ModelSerializer):
+        class Meta:
+            model = CustomPrimaryKeyModel
+            fields = ('char_field',)
+
+    def test_create_method(self):
+        serializer = self.TestSerializer(data={
+            'char_field': 'foo'
+        })
+        assert serializer.is_valid()
+        instance = serializer.save()
+        assert 'foo' == instance.pk
+
+    def test_pk_read_only_on_update(self):
+        instance = CustomPrimaryKeyModel.objects.create(
+            char_field='foo'
+        )
+        instance.save()
+
+        serializer = self.TestSerializer(instance=instance, data={
+            'char_field': 'bar'
+        })
+        assert serializer.is_valid()
+        updated_instance = serializer.save()
+        assert 'bar' != updated_instance.pk
+
+
 class TestRegularFieldMappings(TestCase):
     def test_regular_fields(self):
         """