django - シグナル内の別のモデルからフィールドにアクセスするにはどうすればよいですか?

Question

path_afterシグナル内のフィールドにアクセスするにはどうすればよいですか?

モデル:

class Store(models.Model):
    user = models.ForeignKey(User)
    path = models.FileField(upload_to="src")
    path_after = models.FileField(upload_to="files",blank=True, null=True)

class Document(models.Model):
    rootfile = models.ForeignKey(Store)
    content = models.TextField()

信号：

@receiver(post_save, sender=Store)
def my_handler(sender, **kwargs):
    if kwargs['created']:
        document = Document()
        document.rootfile = kwargs['instance']

        #how to open file from path_after field?
        text_file = open("path_after", "r")

        document.content = #should be text from path_after file, this is TextField
        document.save()