Can't update backref record?
But if i do this , it works.
```
t = Topic.get_by_id(420426)
p = t.post
p.content = 'test'
p.save(only=[Post.content])
``` Every time you access `topic.post` using that property you are re-fetching a new instance (and the one you modified is discarded).
```
class Post(BaseModel):
    content = CharField()
    topic = ForeignKeyField(Topic, null=True, backref='Posts')


class Topic(BaseModel):
    @property
    def post(self):
        return self.posts.get()
```
Then if i do update, it won't work
```
t = Topic.get_by_id(420426)
t.post.content = 'test' # post content won't be updated
t.post.save(only=[Post.content]) # returns 0
```

Since `self.posts.get()` returns the the ref record, it supposed to work
    
