这是我的模型。我想用copy函数从我的模型中复制一份。并将created_time更新到这个时间，最终返回id后的内容。

from django.db import models
from django.utils import timezone

class Author(models.Model):
    name = models.CharField(max_length=50)

class BlogPost(models.Model):
    title = models.CharField(max_length=250)
    body = models.TextField()
    author = models.ForeignKey(Author, on_delete=models.CASCADE)
    date_created = models.DateTimeField(auto_now_add=True)

    def copy(self):
        blog = BlogPost.objects.get(pk=self.pk)
        comments = blog.comment_set.all()

        blog.pk = None
        blog.save()

        for comment in comments:
            comment.pk = None
            comment.blog_post = blog
            comment.save()
        return blog.id

class Comment(models.Model):
    blog_post = models.ForeignKey(BlogPost, on_delete=models.CASCADE)
    text = models.CharField(max_length=500)

我还希望复制功能，使从职位和评论的副本，你能帮我纠正我的代码和更新时间在我的功能。