我最近发现了一些JSON序列化Django中各种对象的代码。不幸的是，代码在遇到某些类型的模型时会抛出AttributeError。
以下是我正在尝试诊断和解决的错误和追溯：
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'
这是序列化程序代码（不是我的，在http://www.traddicts.org/webdevelopment/flexible-and-simple-json-serialization-for-django/找到）
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()
以下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')
以下是我在views.py中执行所有操作的方法：
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')
答案 0 :(得分：3)
我的猜测是GenericRelation的问题。这是一个非常新的字段类型，可能在编写序列化代码时没有实现。
GenericRelation字段在内部使用ManyToMany关系，并且可能以他们未定义field.rel.through对象的方式。这会导致您的错误。您可以通过添加一些日志来验证这一点，以查看哪个字段导致问题。
要解决此问题，您可以查看更多GenericRelations并修改序列化代码以支持它们，或使用支持GenericRelations的其他序列化技术。
从GenericRelation类的Django源代码剪辑：
def get_internal_type(self):
return "ManyToManyField"