为什么我的django模型会抛出错误？

Question

I'm putting together a model for a blog app.

Here's the model:

from django.db import models

class Tag(models.Model):
    keyword = models.CharField(max_length=256)
    posts = models.ManyToManyField(Post)

    def __unicode__(self):
        return self.keyword

class Post(models.Model):
    title = models.CharField(max_length=512)
    image = models.ImageField
    body = models.TextField()
    visible = models.BooleanField()
    date_created = models.DateTimeField(auto_now_add=True)
    date_updated = models.DateTimeField(auto_now=True)
    tags = models.ManyToManyField(Tag)

    def __unicode__(self):
        return self.title

I added the blog package to settings.py. Then I ran python manage.py sql blog . I got the following errors:

 File "/pathto/blog/models.py", line 5, in Tag
    posts = models.ManyToManyField(Post)
NameError: name 'Post' is not defined

I don't understand why Post is not defined because I am defining it in the models.py file. What am I missing?

Answer 1

Problem

When you are defining Tag class, Post class is not yet defined. Because you are referring to it, you get NameError exception.

Solution

Thus, change this line:

posts = models.ManyToManyField(Post)

into this line:

posts = models.ManyToManyField('Post')

Explanation

The documentation gives you walkaround:

If you need to create a relationship on a model that has not yet been defined, you can use the name of the model, rather than the model object itself:

 class Car(models.Model): manufacturer = models.ForeignKey('Manufacturer') # ... class Manufacturer(models.Model): # ... 

Alternative solution

You can also omit definition of posts in Tag class (the line " posts = models.ManyToManyField(Post) ") and just provide the appropriate name for reverse relation in Post model. Django will know what to do with it. Just replace this line:

tags = models.ManyToManyField(Tag)

with this line:

tags = models.ManyToManyField(Tag, related_name='posts')

To learn more, read about related_name argument when defining relations ( ForeignKey and ManyToManyField ).

Answer 2

As answered before , you have are referencing a model (Post) before you've actually defined it -- definition order in a Python file is significant.

To get around the problem, let Django resolve that name rather than Python, by enclosing it with single-quotes:

posts = models.ManyToManyField('Post')