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Updating new fields in my model

I am trying to update new fields in my database about my "card" model, which already had the fields above, but I have a problem that prevents me from doing this process:

When I started. /manage.py syncdb, I received this message:

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.

So, I ran the makemigrations command, but ...

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

I chose to click the second option and add this requirement myself, in fact I have this:

models.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

If I change the "Imagen" field as the message said, I would do the following:

imagen = models.ImageField (upload_to = url, default = '')

But then the same message appears after making the same modification in the "imagen" field:

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:

And finally, the last:

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:

If I change all of these fields, I can finally run. /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

. /manage.py syncdb, :

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

? , :

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()

apologizeme , -.

!!

+4
2

DecimalField Decimal.

from decimal import Decimal

class card(models.Model):
    # ...
    imagen = models.ImageField(upload_to=url, default='')
    precio = models.DecimalField(max_digits=6, decimal_places=2, default=Decimal(0))
    stock = models.IntegerField(default=0)
+4

model.fixed, django_migrations , .

0

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