将 db varchar 映射到域对象 bool

Question

如何处理数据库中的字符串到域模型中的布尔值的映射。考虑一个例子，将抛硬币存储在数据库中。对于通常只有两个值的情况，例如 heads 或 tails。

^{（注意：我意识到在大多数情况下最好在数据库中存储布尔值，但认为这是一个学术练习）}

我希望模型中的 True 在数据库中最终为 "heads"，并且 "tails" 在数据库中最终在模型中为 False。

from sqlalchemy.orm import mapper, sessionmaker
from sqlalchemy import Table, MetaData, Column, Integer, String, create_engine
from unittest import TestCase

class Foo(object):
    def __init__(self, id, is_heads):
        self.id = id
        self.is_heads = is_heads

metadata = MetaData()
db_foo = Table(
    'foo', metadata,
    Column('id', Integer, primary_key=True),
    Column('side', String(5), nullable=False)) # heads or tails

def data_map():
    # what do I need to do here?
    mapper(Foo, db_foo)

class TestMaps(TestCase):
    def test_map(self):
        # Setup
        engine = create_engine('sqlite:///:memory:', echo=True)
        metadata.create_all(engine, tables=[db_foo])
        Session = sessionmaker(bind=engine)
        session = Session()
        data_map()
        expected = Foo(1, False)    
        # Exercise
        session.add(expected)
        session.flush()
        actual = session.query(Foo).first()    
        # Verify
        self.assertEqual(expected, actual)

Answer 1

查看文档的 Augmenting Existing Types 部分。以下应该可以解决问题：

import sqlalchemy.types as types


class CoinFlipDataType(types.TypeDecorator):
    impl = types.String

    def process_bind_param(self, value, dialect):
        if value is None:
            return None
        assert value in (True, False)
        return 'heads' if value else 'tails'

    def process_result_value(self, value, dialect):
        if value is None:
            return None
        assert value in ('heads', 'tails'), value
        return value == 'heads'

    def copy(self):
        return CoinFlipDataType(self.impl.length)

def data_map():
    mapper(Foo, db_foo, properties={'is_heads': db_foo.c.side})

db_foo = Table(
    'foo', metadata,
    Column('id', Integer, primary_key=True),
    Column('side', CoinFlipDataType(5), nullable=False))

将 db varchar 映射到域对象 bool

Mapping db varchar to domain object bool

python

sqlalchemy