更新我的mod中的新字段

Question

我正在尝试更新数据库中有关已具有上述字段的“卡”模型的新字段，但有一个问题阻碍了我执行此过程：

当我运行./manage.py syncdb时，我收到以下消息：

Your models have changes that are not yet reflected in a migration, and so won't be applied.
  Run 'manage.py makemigrations' to make new migrations, and then re-run 'manage.py migrate' to apply them.

所以我运行了makemigrations命令但是。。。

You are trying to add a non-nullable field 'imagen' to card without a default;
we can't do that (the database needs something to populate existing rows).
Please select a fix:
 1) Provide a one-off default now (will be set on all existing rows)
 2) Quit, and let me add a default in models.py

我选择了按第二个选项并自己添加要求，实际上我有：

型号.py：

from django.db import models

class subscriber(models.Model):
    nombre = models.CharField(max_length=200)
    apellidos = models.CharField(max_length=200)
    status = models.BooleanField(default=True)

    def __unicode__(self):
        nombreCompleto = "%s %s"%(self.nombre,self.apellidos)
        return nombreCompleto

def url(self,filename):
    ruta = "MultimediaData/Card/%s/%s"%(self.nombre,str(filename))
    return ruta

class card(models.Model):

    nombre = models.CharField(max_length=100)
    descripcion = models.TextField(max_length=300)
    status = models.BooleanField(default=True)
    imagen = models.ImageField(upload_to=url)
    precio = models.DecimalField(max_digits=6,decimal_places=2)
    stock = models.IntegerField()

    def __unicode__(self):
        return self.nombre

如果我修改“Imagen”字段，如前所述，我将执行以下操作：

imagen=models.ImageField（上传到=url，默认值=“”）

但是，在对“imagen”字段进行相同的修改之后，会出现相同的消息：

You are trying to add a non-nullable field 'precio' to card without a default;
we can't do that (the database needs something to populate existing rows).
Please select a fix:

最后一点：

You are trying to add a non-nullable field 'stock' to card without a default;
we can't do that (the database needs something to populate existing rows).
Please select a fix:

如果修改所有这些字段，我最终可以运行./manage.py makemigrations：

Migrations for 'synopticup':
  0002_auto_20141016_2004.py:
    - Add field imagen to card
    - Add field precio to card
    - Add field stock to card

但是，当我运行./manage.py syncdb时，会得到以下错误：

django.core.exceptions.ValidationError: [u"'' value must be a decimal number."]

我的流程怎么了？我宁愿一切都像以前一样：

class card(models.Model):

    nombre = models.CharField(max_length=100)
    descripcion = models.TextField(max_length=300)
    status = models.BooleanField(default=True)
    imagen = models.ImageField(upload_to=url)
    precio = models.DecimalField(max_digits=6,decimal_places=2)
    stock = models.IntegerField()

如果我忽略了什么，请提前向我道歉。

谢谢！！