python - Django jsonfield 在循环中保留旧对象值

Question

这是模型:

class ModelA(models.Model):
    field1 = models.CharField(max_length=100)
    field2 = models.CharField(max_length=100)
    field3 = JSONField(default=[])

    def save(self, *args, **kwargs):
        # Below print should be None
        # But it shows the value of the previously created object, why ?
        print "------------------------------------------------"
        print self.id
        print "self.field3"
        print self.field3
        self.field2 = self.field1 + " world"
        self.field3.append(self.field2)
        super(ModelA, self).save(*args, **kwargs)

这是 View :

def view1(request):
    for x in range(1, 3):
          a = ModelA.objects.create(field1="hello%s" % x)

预期输出:

# None
# self.field3
# []

# None
# self.field3
# []

实现的输出:

# None
# self.field3
# []

# None
# self.field3
# [u'Hello1 world']

# None
# self.field3
# [u'Hello1 world', u'Hello2 world']

那么，根据给定的输出，你能告诉我为什么它在创建新对象时使用以前的对象值吗？

Answer 1

这是来自 Django documentation :

If you give the field a default, ensure it’s a callable such as dict (for an empty default) or a callable that returns a dict (such as a function). Incorrectly using default={} creates a mutable default that is shared between all instances of JSONField.

因此请使用它:

field3 = JSONField(default=list)