Pycharm can't auto complete some modules

Pycharm works well at most time. But it can't auto complete some code.

The following "objects" can't be completed. Who knows why?

I use Pycharm Community Edition and Django 2.2

class SnippetList(APIView):
    """
    List all snippets, or create a new snippet.
    """

    def get(self, request):
        snippets = Snippet.objects.all()  # The objects can't be auto completed.
        serializer = SnippetSerializer(snippets, many=True)
        return Response(resp)

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