尝试从基础模型生成对象集合

Question

我试图为tinydb制作一个小小的orm但是遇到了类和实例集合的问题：

使用以下代码：

WHERE

这有效：

class SubModelChild(object):

    def __init__(self):
        print "IN INIT OF SubModelChild"

    def cmd2(self):
        print "IN cmd2 of SubModelChild"

    def search(self):
        print "IN search of SubModelChild"
        result = []
        for x in range(0, 3):
            rec = {'id': x, 'text': 'text %d' % x}
            result.append(rec)
        return result

class SubModelMaster(object):
    _child = None

    def __init__(self):
        print "IN INIT OF SubModelMaster"
        self._child = SubModelChild()

    def cmd1(self):
        print "IN cmd1 of SubModelMaster"


class BaseModel(object):
    _store = None
    _em = None

    def __init__(self):
        print "IN INIT OF BaseModel"
        self._store =  SubModelMaster()
        self._em = self._store._child

    @classmethod
    def objects(cls):
        return Manager(cls)

    def cmd3(self):
        print "IN cmd3 of BaseModel"


class Manager(object):
    def __init__(self, model_class):
        self.model_class = model_class
        self._em = model_class._em

    def _model(self, result):
        if result is None: return None
        instance = self.model_class(**result)
        return instance

    def find(self):
        print "IN find of Manager"
        return list(map(self._model, self._em.search()))

class Address(BaseModel):
    address = None
    city = None

# Works
adr = Address()
adr._store.cmd1()
adr._em.cmd2()
adr.cmd3()
for rec in adr._em.search():
    print rec

# Don't works
collection = Address.objects().find()
for rec in collection.find():
    print rec

而这不是：

adr = Address()
adr._store.cmd1()
adr._em.cmd2()
adr.cmd3()
for rec in adr._em.search():
    print rec

确认此错误：

collection = Address.objects().find()
for rec in collection.find():
    print rec

有人可以帮我找到解决方案吗？