如何使用 Pony ORM 存储 Python 枚举？

Question

假设我在这里有这个简单的小 Pony ORM 映射。内置的 Enum 类是 Python 3.4 的新功能，并向后移植到 2.7。

from enum import Enum

from pony.orm import Database, Required


class State(Enum):
    ready = 0
    running = 1
    errored = 2

if __name__ == '__main__':
    db = Database('sqlite', ':memory:', create_db=True)

    class StateTable(db.Entity):
        state = Required(State)

    db.generate_mapping(create_tables=True)

当我运行程序时，会抛出一个错误。

TypeError: No database converter found for type <enum 'State'>

这是因为 Pony 不支持映射枚举类型。当然，这里的解决方法是只存储 Enum 值，并在 Class StateTable 中提供一个 getter 以再次将值转换为 Enum。但这是乏味且容易出错的。我也可以只使用另一个 ORM。如果这个问题变得太令人头疼，也许我会。但如果可以的话，我宁愿坚持使用 Pony。

我更愿意创建一个数据库转换器来存储枚举，就像错误消息所暗示的那样。有谁知道怎么做？

更新： 感谢 Ethan 的帮助，我想出了以下解决方案。

from enum import Enum

from pony.orm import Database, Required, db_session
from pony.orm.dbapiprovider import StrConverter


class State(Enum):
    ready = 0
    running = 1
    errored = 2

class EnumConverter(StrConverter):

    def validate(self, val):
        if not isinstance(val, Enum):
            raise ValueError('Must be an Enum.  Got {}'.format(type(val)))
        return val

    def py2sql(self, val):
        return val.name

    def sql2py(self, value):
        # Any enum type can be used, so py_type ensures the correct one is used to create the enum instance
        return self.py_type[value]

if __name__ == '__main__':
    db = Database('sqlite', ':memory:', create_db=True)

    # Register the type converter with the database
    db.provider.converter_classes.append((Enum, EnumConverter))

    class StateTable(db.Entity):
        state = Required(State)

    db.generate_mapping(create_tables=True)

    with db_session:
        s = StateTable(state=State.ready)
        print('Got {} from db'.format(s.state))

Answer 1

Excerpt from some random mailing list:

2.2。转换方法

每个转换器类都应该定义以下方法：

class MySpecificConverter(Converter):

    def init(self, kwargs):
        # Override this method to process additional positional
        # and keyword arguments of the attribute

       if self.attr is not None:
            # self.attr.args can be analyzed here
            self.args = self.attr.args

        self.my_optional_argument = kwargs.pop("kwarg_name")
        # You should take all valid options from this kwargs
        # What is left in is regarded as unrecognized option

    def validate(self, val):
        # convert value to the necessary type (e.g. from string)
        # validate all necessary constraints (e.g. min/max bounds)
        return val

    def py2sql(self, val):
        # prepare the value (if necessary) to storing in the database
        return val

    def sql2py(self, value):
        # convert value (if necessary) after the reading from the db
        return val

    def sql_type(self):
        # generate corresponding SQL type, based on attribute options
        return "SOME_SQL_TYPE_DEFINITION"

你可以研究一下现有转换器的代码，看看这些方法如何 已实施。