SQLModel中将description转化为数据表定义中的comment

在使用 SQLModel 进行模型定义的时候，使用 description 属性来定义一个 Field 的描述信息。不过在数据库中，用的是 comment。SQLModel 在生成建表语句的时候，默认不会生成 comment 信息，需要使用 sa_column_kwargs={“comment”: “字段说明信息”} 来单独定义 comment，这样会在一个字段的定义上有两个重复的定义。在 https://github.com/tiangolo/sqlmodel/issues/492 这个 issue 中有人写了一个转换程序，摘抄如下：

# Automatically set comment to Column using description.
# https://github.com/tiangolo/sqlmodel/issues/492
class DescriptionMeta(SQLModelMetaclass):
    """ 将 description作为字段的comment """
    def __new__(
        cls,
        name: str,
        bases: Tuple[Type[Any], ...],
        class_dict: Dict[str, Any],
        **kwargs: Any,
    ) -> Any:
        new_class = super().__new__(cls, name, bases, class_dict, **kwargs)
        fields = new_class.model_fields
        for k, field in fields.items():
            desc = field.description
            if desc:
                # deal with sa_column_kwargs
                if field.sa_column_kwargs is not PydanticUndefined:
                    field.sa_column_kwargs["comment"] = desc
                else:
                    field.sa_column_kwargs = {"comment": desc}
                # deal with sa_column
                if field.sa_column is not PydanticUndefined:
                    if not field.sa_column.comment:
                        field.sa_column.comment = desc
                # deal with attributes of new_class
                if hasattr(new_class, k):
                    column = getattr(new_class, k)
                    if hasattr(column, "comment") and not column.comment:
                        column.comment = desc
        return new_class