Can't refer to model which not exists yet
By default you can use topic.post_set.
If you change  topic = ForeignKeyField(Topic) to topic = ForeignKeyField(Topic,related_name='posts')
you can use topic.posts.
 Unfortunately peewee does not support this kind of relationship.  This is partly a limitation of python, but also in part due to my own feeling that these kinds of circular dependencies indicate a problem with the schema.

To work around you really only have one option:

``` python
class Topic(Model):
    title = CharField()
    last_post_id = IntegerField(null=True)

    @property
    def last_post(self):
        return Post.get(Post.id == self.last_post_id)

class Post(Model):
    topic = ForeignKeyField(Topic)
    body = CharField()
```

``` python
class Topic(Model):
    title = CharField()
    last_post = ForeignKeyField(Post)

class Post(Model):
    topic = ForeignKeyField(Topic)
    body = CharField()
```

How to refer 'Post' model from 'Topic' model?
