How to get BaseModel Fields?
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```python

User._meta.fields  # dict of field name -> field instance
User._meta.sorted_fields  # list of field instances in order
User._meta.sorted_field_names  # Just the names
``` Thanks.
I want to get the defined `Fields` as a list.

```
class User(BaseModel):
    id = BigIntegerField(primary_key=True)
    nick = CharField(unique=True)
    age = SmallIntegerField(unique=True)

    class Meta:
        database = db
```