choice field in django rest framework code example

Example: serializermethodfield value is notbeen updated in django model

class SimulationSerializer(serializers.ModelSerializer):
    machine = serializers.PrimaryKeyRelatedField(queryset=Machine.objects.all())
    project = serializers.PrimaryKeyRelatedField(queryset=Project.objects.all())

    class Meta:
        model = Simulation
        fields = ('project', 'machine')