有几个关键的区别。
update 用于查询集，因此可以一次更新多个对象。
正如@FallenAngel 所指出的，自定义save() 方法的触发方式存在差异，但请记住signals 和ModelManagers 也很重要。我已经构建了一个小型测试应用程序来展示一些有价值的差异。我使用的是 Python 2.7.5、Django==1.7.7 和 SQLite，注意最终的 SQL 可能会因 Django 的不同版本和不同的数据库引擎而异。
好的，这是示例代码。
models.py:
from __future__ import print_function
from django.db import models
from django.db.models import signals
from django.db.models.signals import pre_save, post_save
from django.dispatch import receiver
__author__ = 'sobolevn'
class CustomManager(models.Manager):
def get_queryset(self):
super_query = super(models.Manager, self).get_queryset()
print('Manager is called', super_query)
return super_query
class ExtraObject(models.Model):
name = models.CharField(max_length=30)
def __unicode__(self):
return self.name
class TestModel(models.Model):
name = models.CharField(max_length=30)
key = models.ForeignKey('ExtraObject')
many = models.ManyToManyField('ExtraObject', related_name='extras')
objects = CustomManager()
def save(self, *args, **kwargs):
print('save() is called.')
super(TestModel, self).save(*args, **kwargs)
def __unicode__(self):
# Never do such things (access by foreing key) in real life,
# because it hits the database.
return u'{} {} {}'.format(self.name, self.key.name, self.many.count())
@receiver(pre_save, sender=TestModel)
@receiver(post_save, sender=TestModel)
def reicever(*args, **kwargs):
print('signal dispatched')
views.py:
def index(request):
if request and request.method == 'GET':
from models import ExtraObject, TestModel
# Create exmple data if table is empty:
if TestModel.objects.count() == 0:
for i in range(15):
extra = ExtraObject.objects.create(name=str(i))
test = TestModel.objects.create(key=extra, name='test_%d' % i)
test.many.add(test)
print test
to_edit = TestModel.objects.get(id=1)
to_edit.name = 'edited_test'
to_edit.key = ExtraObject.objects.create(name='new_for')
to_edit.save()
new_key = ExtraObject.objects.create(name='new_for_update')
to_update = TestModel.objects.filter(id=2).update(name='updated_name', key=new_key)
# return any kind of HttpResponse
这导致了这些 SQL 查询：
# to_edit = TestModel.objects.get(id=1):
QUERY = u'SELECT "main_testmodel"."id", "main_testmodel"."name", "main_testmodel"."key_id"
FROM "main_testmodel"
WHERE "main_testmodel"."id" = %s LIMIT 21'
- PARAMS = (u'1',)
# to_edit.save():
QUERY = u'UPDATE "main_testmodel" SET "name" = %s, "key_id" = %s
WHERE "main_testmodel"."id" = %s'
- PARAMS = (u"'edited_test'", u'2', u'1')
# to_update = TestModel.objects.filter(id=2).update(name='updated_name', key=new_key):
QUERY = u'UPDATE "main_testmodel" SET "name" = %s, "key_id" = %s
WHERE "main_testmodel"."id" = %s'
- PARAMS = (u"'updated_name'", u'3', u'2')
update() 只有一个查询，save() 有两个查询。
接下来，让我们谈谈重写save() 方法。 save() 方法显然只调用一次。值得一提的是，.objects.create() 也调用了save() 方法。
但update() 不会在模型上调用save()。如果没有为update() 调用save() 方法，则也不会触发信号。输出：
Starting development server at http://127.0.0.1:8000/
Quit the server with CONTROL-C.
# TestModel.objects.get(id=1):
Manager is called [<TestModel: edited_test new_for 0>]
Manager is called [<TestModel: edited_test new_for 0>]
save() is called.
signal dispatched
signal dispatched
# to_update = TestModel.objects.filter(id=2).update(name='updated_name', key=new_key):
Manager is called [<TestModel: edited_test new_for 0>]
如您所见，save() 触发了Manager 的get_queryset() 两次。当update() 只有一次。
分辨率。如果您需要“静默”更新您的值，而无需调用 save() - 请使用 update。用例：last_seen 用户字段。当您需要正确更新模型时，请使用save()。