from django_http_last_modified_view.views import HttpLastModifiedMixin

class MyView(HttpLastModifiedMixin,...):

    def get_http_last_modified(self):
        return self.obj.http_last_modified

class MyView(HttpLastModifiedMixin,...):
    def dispatch(self, *args, **kwargs):
        self.http_last_modified = obj.last_modified_at
        return super().dispatch(*args, **kwargs)