Google App Engine模型的自定义键（Python）

Question

首先，我对Google App Engine还比较陌生，所以可能做了一些傻事。

假设我有一个模型叫做Foo：

class Foo(db.Model):
   name = db.StringProperty()

我想用name作为每个Foo对象的唯一标识。这个怎么实现呢？

现在，当我想获取一个特定的Foo对象时，我会在数据存储中查询所有具有目标唯一名称的Foo对象，但查询速度很慢（而且在每次创建新的Foo时，确保name是唯一的也很麻烦）。

肯定有更好的方法来做到这一点！

谢谢。

Answer 1

这里有一篇关于AppEngine数据存储中唯一性问题的详细讨论：我该如何为Google App Engine中的模型定义一个唯一属性？

Answer 2

我之前在一个项目中用过下面的代码。只要你用来生成键名的字段是必填的，它就能正常工作。

class NamedModel(db.Model):
    """A Model subclass for entities which automatically generate their own key
    names on creation. See documentation for _generate_key function for
    requirements."""

    def __init__(self, *args, **kwargs):
        kwargs['key_name'] = _generate_key(self, kwargs)
        super(NamedModel, self).__init__(*args, **kwargs)


def _generate_key(entity, kwargs):
    """Generates a key name for the given entity, which was constructed with
    the given keyword args.  The entity must have a KEY_NAME property, which
    can either be a string or a callable.

    If KEY_NAME is a string, the keyword args are interpolated into it.  If
    it's a callable, it is called, with the keyword args passed to it as a
    single dict."""

    # Make sure the class has its KEY_NAME property set
    if not hasattr(entity, 'KEY_NAME'):
        raise RuntimeError, '%s entity missing KEY_NAME property' % (
            entity.entity_type())

    # Make a copy of the kwargs dict, so any modifications down the line don't
    # hurt anything
    kwargs = dict(kwargs)

    # The KEY_NAME must either be a callable or a string.  If it's a callable,
    # we call it with the given keyword args.
    if callable(entity.KEY_NAME):
        return entity.KEY_NAME(kwargs)

    # If it's a string, we just interpolate the keyword args into the string,
    # ensuring that this results in a different string.
    elif isinstance(entity.KEY_NAME, basestring):
        # Try to create the key name, catching any key errors arising from the
        # string interpolation
        try:
            key_name = entity.KEY_NAME % kwargs
        except KeyError:
            raise RuntimeError, 'Missing keys required by %s entity\'s KEY_NAME '\
                'property (got %r)' % (entity.entity_type(), kwargs)

        # Make sure the generated key name is actually different from the
        # template
        if key_name == entity.KEY_NAME:
            raise RuntimeError, 'Key name generated for %s entity is same as '\
                'KEY_NAME template' % entity.entity_type()

        return key_name

    # Otherwise, the KEY_NAME is invalid
    else:
        raise TypeError, 'KEY_NAME of %s must be a string or callable' % (
            entity.entity_type())

然后你可以像这样修改你的示例模型：

class Foo(NamedModel):
    KEY_NAME = '%(name)s'
    name = db.StringProperty()

当然，在你的情况下，这可以大大简化，只需把NamedModel的__init__方法的第一行改成类似这样的内容：

kwargs['key_name'] = kwargs['name']