Cause a error when using join
Why is `category_id` not a `ForeignKeyField`?

```python
class CsArticle(MyBaseModel):
    ...
    category = ForeignKeyField(Category)  # This will work instead of category_id field.
```

After that, your ".alias()" needs to apply to the "on" expression...not the query as a whole:

```python
.join(
    Category,
    on=(CsArticle.category_id == Category.id).alias('cate'))  # Note parentheses.
```
## These are my models:
```python
class MyBaseModel(Model):
    class Meta:
        database = db
class Category(MyBaseModel):
    category_name = CharField(null=False, unique=True)
class CsArticle(MyBaseModel):
    title = CharField(null=False, unique=True)
    md = TextField(null=False)
    html = TextField(null=False)
    date = DateField(null=False)
    category_id = IntegerField(null=False)
    number = IntegerField(null=False, default=0)
```

## When I do this query:
```python
datas = ( CsArticle
                .select(CsArticle, Category)
                .join(
                    Category,
                    on=(CsArticle.category_id == Category.id)).alias('cate')
                .order_by(CsArticle.date.desc()))
response.append({'title': data.title,
                             'category': data.cate.category_name,
                             'date': data.date,
                             'id': data.id, 'number': data.number})
```
## It's caused such error:
`File "/python-code/kunkun_blog/reader_request/article_list.py", line 44, in _get_cs_list
    'category': data.cate.category_name,
AttributeError: 'CsArticle' object has no attribute 'cate'`

