Dynamically limiting queryset of related field

Using Django REST Framework, I want to limit which values can be used in a related field in a creation.

For example consider this example (based on the filtering example on https://web.archive.org/web/20140515203013/http://www.django-rest-framework.org/api-guide/filtering.html, but changed to ListCreateAPIView):

class PurchaseList(generics.ListCreateAPIView)
    model = Purchase
    serializer_class = PurchaseSerializer

    def get_queryset(self):
        user = self.request.user
        return Purchase.objects.filter(purchaser=user)

In this example, how do I ensure that on creation the purchaser may only be equal to self.request.user, and that this is the only value populated in the dropdown in the form in the browsable API renderer?