无法弄清楚为什么Django不能保存模型字段

Question

我在其他地方找到了一些代码，以便在有现有电子邮件帐户时让django-rest-auth与社交登录一起使用：

def pre_social_login(self, request, sociallogin):
    """
    Invoked just after a user successfully authenticates via a
    social provider, but before the login is actually processed
    (and before the pre_social_login signal is emitted).

    We're trying to solve different use cases:
    - social account already exists, just go on
    - social account has no email or email is unknown, just go on
    - social account's email exists, link social account to existing user
    """

    # Ignore existing social accounts, just do this stuff for new ones
    if sociallogin.is_existing:
        return

    # some social logins don't have an email address, e.g. facebook accounts
    # with mobile numbers only, but allauth takes care of this case so just
    # ignore it
    if 'email' not in sociallogin.account.extra_data:
        return

    # check if given email address already exists.
    # Note: __iexact is used to ignore cases
    try:
        email = sociallogin.account.extra_data['email'].lower()
        id = sociallogin.account.extra_data['id']
        email_address = EmailAddress.objects.get(email__iexact=email)


    # if it does not, let allauth take care of this new social account
    except EmailAddress.DoesNotExist:
        return

    # if it does, connect this new social login to the existing user
    user = email_address.user
    profile = Profile.objects.get(email__iexact=email) 
    url = "http://graph.facebook.com/{}/picture?type=normal".format(id)
    profile.media_url = url 
    profile.save() # suppose to persist to db, but isn't!!!
    sociallogin.connect(request, user)

在连接帐户之前的最后几行中，我获得了FB帐户的个人资料图片并尝试将其保存到数据库中。对于我的生活，我无法理解为什么它不会保存网址。如果我使用django shell执行相同的命令，它会保存没有问题。如果有帮助，media_url字段为URLField。