Django model_to_dict的一个问题

自定义model_to_dict()方法，在model里面加上一个方法

def to_dict(self):
       opts = self._meta
       data = {}
       for f in opts.concrete_fields:
           value = f.value_from_object(self)
           if isinstance(value, datetime):
               value = value.strftime('%Y-%m-%d %H:%M:%S')
           elif isinstance(f, FileField):
               value = value.url if value else None
           data[f.name] = value
       return data

"""
Why is `__fields` in here?
    it holds the list of fields except for the one ends with a suffix '__[field_name]'.
    When converting a model object to a dictionary using this method,
    You can use a suffix to point to the field of ManyToManyField in the model instance.
    The suffix ends with '__[field_name]' like 'publications__name'
"""
__fields = list(map(lambda a: a.split('__')[0], fields or []))

for f in chain(opts.concrete_fields, opts.virtual_fields, opts.many_to_many):
    is_edtiable = getattr(f, 'editable', False)

    if fields and f.name not in __fields:
        continue

    if exclude and f.name in exclude:
        continue

    if isinstance(f, ManyToManyField):
        if instance.pk is None:
            data[f.name] = []
        else:
            qs = f.value_from_object(instance)
            if qs._result_cache is not None:
                data[f.name] = [item.pk for item in qs]
            else:
                try:
                    m2m_field  = list(filter(lambda a: f.name in a and a.find('__') != -1, fields))[0]
                    key = m2m_field[len(f.name) + 2:]
                    data[f.name] = list(qs.values_list(key, flat=True))
                except IndexError:
                    data[f.name] = list(qs.values_list('pk', flat=True))

    elif isinstance(f, DateTimeField):
        date = f.value_from_object(instance)
        data[f.name] = date_to_strf(date) if date_to_strf else date_to_timestamp(date)

    elif isinstance(f, ImageField):
        image = f.value_from_object(instance)
        data[f.name] = image.url if image else None

    elif isinstance(f, FileField):
        file = f.value_from_object(instance)
        data[f.name] = file.url if file  else None

    elif is_edtiable:
        data[f.name] = f.value_from_object(instance)

"""
Just call an instance's function or property from a string with the function name in `__fields` arguments.
"""
funcs = set(__fields) - set(list(data.keys()))
for func in funcs:
    obj = getattr(instance, func)
    if inspect.ismethod(obj):
        data[func] = obj()
    else:
        data[func] = obj
return data