Django视图代码有奇怪的行为

Question

我写了这个Django视图方法：

def list_all_devices(request):

    all_devices_info = []
    fields = Devices.objects.all()
    for field in fields:
        device = Devices.objects.filter(device_id=field.device_id).values()
        dev = device[0]

        dev['variables'] = Variables.objects.filter(device_id=field.device_id).values()

        del dev['device_creation_datetime']
        all_devices_info.append(dev)

    return HttpResponse(json.dumps(all_devices_info), content_type="application/json")

由于某种原因，第一个代码不起作用，但如果我做了如下的小修改，它就会开始工作，我无法弄清楚原因。

def list_all_devices(request):

    all_devices_info = []
    fields = Devices.objects.all()
    for field in fields:
        device = Devices.objects.filter(device_id=field.device_id).values()
        dev = device[0]

        dev['variables'] = [v for v in Variables.objects.filter(device_id=field.device_id).values()]

        del dev['device_creation_datetime']
        all_devices_info.append(dev)

    return HttpResponse(json.dumps(all_devices_info), content_type="application/json")

当我分析修改时：

[v for v in Variables.objects.filter(device_id=field.device_id).values()]

和

Variables.objects.filter(device_id=field.device_id).values()

它们似乎很相似，似乎都是列表，内部也有相同的信息。但是当调用json.dumps方法时，在第一个代码中它会在浏览器上抛出以下错误。

Answer 1

values（）不会返回简单的数据类型。

我尝试使用django模型：

dir(myModel.objects.all().values())

[＆＃39; 和＆＃39;，＆＃39; bool ＆＃39;，＆＃39; 类 ＆＃39;，＆＃39; deepcopy ＆＃39;，＆＃39; delattr ＆＃39;，＆＃39; dict ＆＃39;，＆＃39; doc ＆＃39;，＆＃39; 格式＆＃39;，＆＃39; getattribute ＆＃39;，＆＃39; getitem ＆＃39;，＆＃39; getstate ＆＃39;，＆＃39; 哈希< / strong>＆＃39;，＆＃39; init ＆＃39;，＆＃39; iter ＆＃39;，＆＃39; len ＆＃39;，＆＃39; 模块＆＃39;，＆＃39; 新＆＃39;，＆＃39; 非零＆＃39;，＆＃39; 或＆＃39;，＆＃39; 减少＆＃39;，＆＃39; reduce_ex ＆＃39;，＆＃39; repr ＆＃39;，＆＃39; setattr ＆＃39;，＆＃39; < strong> setstate ＆＃39;，＆＃39; sizeof ＆＃39;，＆＃39; str ＆＃39;，＆＃39; 子类别＆＃39;，＆＃39; 弱点＆＃39;，＆＃39; _add_hints＆＃39;， ＆＃39; _as_sql＆＃39;，＆＃39; _base_queryset_class＆＃39;，＆＃39; _batched_insert＆＃39;，＆＃39; _clone＆＃39;，＆＃39; _create_object_from_params＆＃39;，＆＃39; _db＆ ＃39;，＆＃39; _earliest_or_latest＆＃39;，＆＃39; _extract_model_params＆＃39;，＆＃39; _fetch_all＆＃39;，＆＃39; _fields＆＃39;，＆＃39; _filter_or_exclude＆＃39;，＆ ＃39; _for_write＆＃39;，＆＃39; _has_filters＆＃39;，＆＃39; _hints＆＃39;，＆＃39; _insert＆＃39;，＆＃39; _known_related_objects＆＃39;，＆＃39; _merge_known_related_objects＆＃ 39;，＆＃39; _merge_sanity_check＆＃39;，＆＃39; _next_is_sticky＆＃39;，＆＃39; _populate_pk_values＆＃39;，＆＃39; _prefetch_done＆＃39;，＆＃39; _prefetch_related_lookups＆＃39;，＆＃ 39; _prefetch_related_objects＆＃39;，＆＃39; _prepare＆＃39;，＆＃39; _raw_delete＆＃39;，＆＃39; _result_cache＆＃39;，＆＃39; _setup_aggregate_query＆＃39;，＆＃39; _setup_query＆＃39 ;，＆＃39; _specialized_queryset_class＆＃39;，＆＃39; _sticky_filter＆＃39;，＆＃39; _update＆＃39;，＆＃39;聚合＆＃39;，＆＃39;所有＆＃39;，＆＃39 ;注释＆＃39;，＆＃39; annotation_names＆＃39; as_manager＆＃39;，＆＃39; bulk_create＆＃39;，＆＃39; complex_filter＆＃39;，＆＃39;计算＆＃39;，＆＃39;创建＆＃39;，＆＃39;日期＆＃39;，＆＃39; datetimes＆＃39;，＆＃39; db＆＃39;，＆＃39; defer＆＃39;， ＆＃39;删除＆＃39;，＆＃39; distinct＆＃39;，＆＃39;最早＆＃39;，＆＃39;排除＆＃39;，＆＃39;存在＆＃39;，＆＃39;额外＆ ＃39;，＆＃39; extra_names＆＃39;，＆＃39; field_names＆＃39;，＆＃39; filter＆＃39;，＆＃39; first＆＃39;，＆＃39; get＆＃39;，＆ ＃39; get_or_create＆＃39;，＆＃39; in_bulk＆＃39;，＆＃39; is_compatible_query_object_type＆＃39;，＆＃39; iterator＆＃39;，＆＃39; last＆＃39;，＆＃39; latest＆＃ 39;，＆＃39; model＆＃39;，＆＃39; none＆＃39;，＆＃39; only＆＃39;，＆＃39; order_by＆＃39;，＆＃39; ordered＆＃39;，＆＃ 39; prefetch_related＆＃39;，＆＃39;查询＆＃39;，＆＃39; raw＆＃39;，＆＃39;反向＆＃39;，＆＃39; select_for_update＆＃39;，＆＃39; select_related＆＃39 ;，＆＃39;更新＆＃39;，＆＃39; update_or_create＆＃39;，＆＃39;使用＆＃39;，＆＃39; value_annotation＆＃39;，＆＃39;值＆＃39;，＆＃39; ; values_list＆＃39;]

这是python数组的目录：

dir([])

[＆＃39; 添加＆＃39;，＆＃39; 类＆＃39;，＆＃39; 包含 ＆＃39;，＆＃39; delattr ＆＃39;，＆＃39; delitem ＆＃39;，＆＃39; delslice ＆＃39;，＆＃39; doc ＆＃39;，＆＃39; eq ＆＃39;，＆＃39; 格式＆＃39;，＆＃39; ge ＆＃39;，＆＃39; getattribute ＆＃39;，＆＃39; getitem < / strong>＆＃39;，＆＃39; getslice ＆＃39;，＆＃39; gt ＆＃39;，＆＃39; 哈希＆＃39;，＆＃39; iadd ＆＃39;，＆＃39; imul ＆＃39;，＆＃39; init ＆＃39;，＆＃39; iter ＆＃39;，＆＃39; le ＆＃39;，＆＃39; len ＆＃39;，＆＃39; lt ＆＃39;，＆＃39; mul ＆＃39;，＆＃39; < strong> ne ＆＃39;，＆＃39; 新＆＃39;，＆＃39; reduce ＆＃39;，＆＃39; reduce_ex ＆＃39;，＆＃39; repr ＆＃39;，＆＃39; 颠倒＆＃39;，＆＃39 ;的 rmul ＆＃39;，＆＃39; setattr ＆＃39;，＆＃39; setitem ＆＃39;，＆＃39; setslice ＆＃39;，＆＃39; sizeof ＆＃39;，＆＃39; str ＆＃39;，＆＃39; < strong>子类分类＆＃39;，＆＃39;追加＆＃39;，＆＃39;计数＆＃39;，＆＃39;延伸＆＃39;，＆＃39;索引＆＃39;，＆＃ 39;插入＆＃39;，＆＃39; pop＆＃39;，＆＃39;删除＆＃39;，＆＃39;反向＆＃39;，＆＃39;排序＆＃39;]

Answer 2

Devices.objects.filter（device_id = field.device_id）.values（）是ValuesQuerySet，它派生自QuerySet，两者都是unevaluated，这意味着这是一个查询在应用列表或迭代之前，不会在数据库上执行。 当您使用json.dumps时，您需要不查询数据。

以下申请查看类型：

Devices.objects.filter(device_id=field.device_id).values().__class__

  

django.db.models.query.ValuesQuerySet

Devices.objects.filter(device_id=field.device_id).values().__class__.__base__

  

django.db.models.query.QuerySet