Django 序列化器错误：“NoneType”对象没有属性“_meta”；

Question

我最近发现了大量用于 JSON 序列化 Django 中各种对象的代码。不幸的是，当代码遇到某些类型的模型时，它会抛出 AttributeError 。

这是我试图诊断和解决的错误和回溯：

AttributeError at /serial/
'NoneType' object has no attribute '_meta'
Request Method: GET
Request URL:    http://127.0.0.1:8000/serial/
Django Version: 1.2.5
Exception Type: AttributeError
Exception Value:    
'NoneType' object has no attribute '_meta'
Exception Location: /Users/jphill/apps/d_projects/smartgoal/../smartgoal/hq/custom_serializer.py in handle_m2m_field, line 183
Python Executable:  /usr/bin/python
Python Version: 2.6.7
Python Path:    ['/Users/jphill/apps/d_projects/smartgoal', '/Library/Python/2.6/site-packages/setuptools-0.6c11-py2.6.egg', '/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python26.zip', '/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6', '/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/plat-darwin', '/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/plat-mac', '/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/plat-mac/lib-scriptpackages', '/System/Library/Frameworks/Python.framework/Versions/2.6/Extras/lib/python', '/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-tk', '/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-old', '/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/lib-dynload', '/Library/Python/2.6/site-packages', '/System/Library/Frameworks/Python.framework/Versions/2.6/Extras/lib/python/PyObjC']
Server time:    Fri, 11 Nov 2011 23:08:13 -0500

Environment:

Request Method: GET
Request URL: http://127.0.0.1:8000/serial/
Django Version: 1.2.5
Python Version: 2.6.7
Installed Applications:
['django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.sites',
 'django.contrib.messages',
 'django.contrib.admin',
 'django.contrib.admindocs',
 'hq']
Installed Middleware:
('django.middleware.common.CommonMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware')


Traceback:
File "/Library/Python/2.6/site-packages/django/core/handlers/base.py" in get_response
  100.                     response = callback(request, *callback_args, **callback_kwargs)
File "/Users/jphill/apps/d_projects/smartgoal/hq/views.py" in serial
  164.      return json_response_from(Task.objects.all())
File "/Users/jphill/apps/d_projects/smartgoal/hq/views.py" in json_response_from
  88.     return HttpResponse(jsonSerializer.serialize(response, ), mimetype='application/json')
File "/Users/jphill/apps/d_projects/smartgoal/../smartgoal/hq/custom_serializer.py" in serialize
  34.         self.handle_object(obj)
File "/Users/jphill/apps/d_projects/smartgoal/../smartgoal/hq/custom_serializer.py" in handle_object
  75.             self.handle_queryset(object)
File "/Users/jphill/apps/d_projects/smartgoal/../smartgoal/hq/custom_serializer.py" in handle_queryset
  138.             self.handle_model(mod)
File "/Users/jphill/apps/d_projects/smartgoal/../smartgoal/hq/custom_serializer.py" in handle_model
  128.                     self.handle_m2m_field(mod, field)
File "/Users/jphill/apps/d_projects/smartgoal/../smartgoal/hq/custom_serializer.py" in handle_m2m_field
  183.         if field.rel.through._meta.auto_created:

Exception Type: AttributeError at /serial/
Exception Value: 'NoneType' object has no attribute '_meta' 

这是序列化器代码（不是我的，在这里找到http://www.traddicts.org/webdevelopment/flexible-and-simple-json-serialization-for-django/）

from io import StringIO
from django.db.models import Model
from django.db.models.query import QuerySet
from django.utils.encoding import smart_unicode
from django.utils.simplejson import dumps

class UnableToSerializeError(Exception):
    """ Error for not implemented classes """
    def __init__(self, value):
        self.value = value
        Exception.__init__(self)

    def __str__(self):
        return repr(self.value)

class JSONSerializer():
    boolean_fields = ['BooleanField', 'NullBooleanField']
    datetime_fields = ['DatetimeField', 'DateField', 'TimeField']
    number_fields = ['IntegerField', 'AutoField', 'DecimalField', 'FloatField', 'PositiveSmallIntegerField']


    def serialize(self, obj, **options):
        self.options = options

        self.stream = options.pop("stream", StringIO())
        self.selectedFields = options.pop("fields", None)
        self.ignoredFields = options.pop("ignored", None)
        self.use_natural_keys = options.pop("use_natural_keys", False)
        self.currentLoc = ''

        self.level = 0

        self.start_serialization()

        self.handle_object(obj)

        self.end_serialization()
        return self.getvalue()

    def get_string_value(self, obj, field):
        """Convert a field's value to a string."""
        return smart_unicode(field.value_to_string(obj))

    def start_serialization(self):
        """Called when serializing of the queryset starts."""
        pass

    def end_serialization(self):
        """Called when serializing of the queryset ends."""
        pass

    def start_array(self):
        """Called when serializing of an array starts."""
        self.stream.write(u'[')
    def end_array(self):
        """Called when serializing of an array ends."""
        self.stream.write(u']')

    def start_object(self):
        """Called when serializing of an object starts."""
        self.stream.write(u'{')

    def end_object(self):
        """Called when serializing of an object ends."""
        self.stream.write(u'}')

    def handle_object(self, object):
        """ Called to handle everything, looks for the correct handling """
        if isinstance(object, dict):
            self.handle_dictionary(object)
        elif isinstance(object, list):
            self.handle_list(object)
        elif isinstance(object, Model):
            self.handle_model(object)
        elif isinstance(object, QuerySet):
            self.handle_queryset(object)
        elif isinstance(object, bool):
            self.handle_simple(object)
        elif isinstance(object, int) or isinstance(object, float) or isinstance(object, long):
            self.handle_simple(object)
        elif isinstance(object, basestring):
            self.handle_simple(object)
        else:
            raise UnableToSerializeError(type(object))

    def handle_dictionary(self, d):
        """Called to handle a Dictionary"""
        i = 0
        self.start_object()
        for key, value in d.iteritems():
            self.currentLoc += key+'.'
            #self.stream.write(unicode(self.currentLoc))
            i += 1
            self.handle_simple(key)
            self.stream.write(u': ')
            self.handle_object(value)
            if i != len(d):
                self.stream.write(u', ')
            self.currentLoc = self.currentLoc[0:(len(self.currentLoc)-len(key)-1)]
        self.end_object()

    def handle_list(self, l):
        """Called to handle a list"""
        self.start_array()

        for value in l:
            self.handle_object(value)
            if l.index(value) != len(l) -1:
                self.stream.write(u', ')

        self.end_array()

    def handle_model(self, mod):
        """Called to handle a django Model"""
        self.start_object()

        for field in mod._meta.local_fields:
            if field.rel is None:
                if self.selectedFields is None or field.attname in self.selectedFields or field.attname:
                    if self.ignoredFields is None or self.currentLoc + field.attname not in self.ignoredFields:
                        self.handle_field(mod, field)
            else:
                if self.selectedFields is None or field.attname[:-3] in self.selectedFields:
                    if self.ignoredFields is None or self.currentLoc + field.attname[:-3] not in self.ignoredFields:
                        self.handle_fk_field(mod, field)
        for field in mod._meta.many_to_many:
            if self.selectedFields is None or field.attname in self.selectedFields:
                if self.ignoredFields is None or self.currentLoc + field.attname not in self.ignoredFields:
                    self.handle_m2m_field(mod, field)
        self.stream.seek(self.stream.tell()-2)
        self.end_object()

    def handle_queryset(self, queryset):
        """Called to handle a django queryset"""
        self.start_array()
        it = 0
        for mod in queryset:
            it += 1
            self.handle_model(mod)
            if queryset.count() != it:
                self.stream.write(u', ')
        self.end_array()

    def handle_field(self, mod, field):
        """Called to handle each individual (non-relational) field on an object."""
        self.handle_simple(field.name)
        if field.get_internal_type() in self.boolean_fields:
            if field.value_to_string(mod) == 'True':
                self.stream.write(u': true')
            elif field.value_to_string(mod) == 'False':
                self.stream.write(u': false')
            else:
                self.stream.write(u': undefined')
        else:
            self.stream.write(u': ')
            self.handle_simple(field.value_to_string(mod))
        self.stream.write(u', ')

    def handle_fk_field(self, mod, field):
        """Called to handle a ForeignKey field."""
        related = getattr(mod, field.name)
        if related is not None:
            if field.rel.field_name == related._meta.pk.name:
                # Related to remote object via primary key
                pk = related._get_pk_val()
            else:
                # Related to remote object via other field
                pk = getattr(related, field.rel.field_name)
            d = {
                    'pk': pk,
                }
            if self.use_natural_keys and hasattr(related, 'natural_key'):
                d.update({'natural_key': related.natural_key()})
            if type(d['pk']) == str and d['pk'].isdigit():
                d.update({'pk': int(d['pk'])})

            self.handle_simple(field.name)
            self.stream.write(u': ')
            self.handle_object(d)
            self.stream.write(u', ')

    def handle_m2m_field(self, mod, field):
        """Called to handle a ManyToManyField."""
        if field.rel.through._meta.auto_created:
            self.handle_simple(field.name)
            self.stream.write(u': ')
            self.start_array()
            hasRelationships = False
            for relobj in getattr(mod, field.name).iterator():
                hasRelationships = True
                pk = relobj._get_pk_val()
                d = {
                        'pk': pk,
                    }
                if self.use_natural_keys and hasattr(relobj, 'natural_key'):
                    d.update({'natural_key': relobj.natural_key()})
                if type(d['pk']) == str and d['pk'].isdigit():
                    d.update({'pk': int(d['pk'])})

                self.handle_simple(d)
                self.stream.write(u', ')
            if hasRelationships:
                self.stream.seek(self.stream.tell()-2)
            self.end_array()
            self.stream.write(u', ')

    def handle_simple(self, simple):
        """ Called to handle values that can be handled via simplejson """
        self.stream.write(unicode(dumps(simple)))

    def getvalue(self):
        """Return the fully serialized object (or None if the output stream is  not seekable).sss """
        if callable(getattr(self.stream, 'getvalue', None)):
            return self.stream.getvalue()

这是我正在尝试序列化的模型：

class Task(models.Model):
    """
    Model used for tracking tasks
    """
    PRIORITY_CHOICES = (
        ('0', 'None'),
        ('1', 'High'),
        ('2', 'Medium'),
        ('3', 'Low'),
    )
    name = models.CharField(max_length=255)
    completed = models.BooleanField(default=False)
    hidden = models.BooleanField(default=False)
    timestamp = models.DateField(auto_now=True)
    priority = models.CharField(default=0, max_length=1, choices=PRIORITY_CHOICES)
    creator = models.ForeignKey(User, related_name="created_task")
    # optional
    owner = models.ForeignKey(User, related_name="owned_task", blank=True, null=True)
    goal = models.ForeignKey('Goal', blank=True, null=True)
    reminder = models.DateTimeField(blank=True, null=True)
    note = models.TextField(blank=True)
    started = models.BooleanField(default=False, blank=True)
    activities = generic.GenericRelation('Activity')

这是我在views.py中执行所有内容的方式：

def serial(request):
     return json_response_from(Task.objects.all())

def json_response_from(response):
    jsonSerializer = JSONSerializer()
    return HttpResponse(jsonSerializer.serialize(response, use_natural_keys=True), mimetype='application/json')

Answer 1

我的猜测是这是 GenericRelation 的问题。这是相当新的字段类型，可能在编写序列化代码时尚未实现。

GenericRelation 字段在内部使用 ManyToMany 关系，并且可能以某种方式不定义 field.rel.through 对象。这会导致您的情况出现错误。您可以通过添加一些日志记录来验证这一点，以查看哪个字段导致了问题。

要解决此问题，您可以更多地了解 GenericRelations 并修改序列化代码以支持它们，或者使用支持 GenericRelations 的其他序列化技术。

从 GenericRelation 类的 Django 源代码中剪辑：

    def get_internal_type(self):
      return "ManyToManyField"