Unable to create model with ManyToManyField
Read the docs: http://docs.peewee-orm.com/en/latest/peewee/relationships.html#manytomany

And: http://docs.peewee-orm.com/en/latest/peewee/relationships.html#manytomanyfield I read the docs many times, but i still did not find what am i doing bad?
Hi, I am using mysql version 5.7.24 and when i try to create Model with ManyToMany relations i got 
error:
```


    raise errorclass(errno, errval)
peewee.IntegrityError: (1048, "Column 'chat_id' cannot be null")
```

Here is my code:

```
class User(BaseModel):
    name = peewee.CharField()
    surname = peewee.CharField()
    email = peewee.CharField()

    def __str__(self):
        return self.surname

lass Message(BaseModel):
    text = peewee.CharField()
    send_by = peewee.ForeignKeyField(User)

    def __str__(self):
        return self.text


class Chat(BaseModel, ManyToManyModel):
    message = ManyToManyField(Message, backref="chats")
    user = ManyToManyField(User, backref="users")


u =  User.get(id=1)
m = Message.get(id=1)
Chat.create(message=m, user=u)
```

I have created tables chat_message_through and chat_user_through.
Thanks for help
