跟踪 NDB 中未知种类的反向引用 [英] Following backreferences of unknown kinds in NDB

问题描述

我正在编写我的第一个基于 GAE 和 Python 2.7 运行时的 RESTful Web 服务；我已经开始使用 Guido 闪亮的新 ndb API.

I'm in the process of writing my first RESTful web service atop GAE and the Python 2.7 runtime; I've started out using Guido's shiny new ndb API.

但是，我不确定如何在没有原始数据库 API 的隐式反向引用功能的情况下解决特定情况.如果用户代理请求特定资源并且这些资源被删除 1 度:

However, I'm unsure how to solve a particular case without the implicit back-reference feature of the original db API. If the user-agent requests a particular resource and those resources 1 degree removed:

host/api/kind/id?depth=2

host/api/kind/id?depth=2

考虑到相关实体的类型在开发时未知，从一对多关系中的一"中发现相关实体集合的最佳方法是什么?

• 我无法使用 之前的 SO 查询.我的模型在运行时可定义(因此不是硬编码)这​​一事实使我无法使用查询来过滤匹配键的属性.

• I'm unable to use a replacement query as described in a previous SO inquiry due to the latter restriction. The fact that my model is definable at runtime (and therefore isn't hardcoded) prevents me from using a query to filter properties for matching keys.

Ancestor 和其他无种类查询也因数据存储限制而无法使用，这使我无法过滤未指定种类的属性.

Ancestor and other kindless queries are also out due to the datastore limitation that prevents me from filtering on a property without the kind specified.

到目前为止，我唯一的想法(除了恢复到 db api)是使用跨组事务在one"上编写我自己的引用，或者通过更新 ndb.StringProperty(repeat=True) 在引入新类型的实体时包含所有相关类型，或者每次将相关的许多"实体写入数据存储区时，只需在一个"ndb.KeyProperty(repeat=True) 上维护一个键列表.

Thus far, the only idea I've had (beyond reverting to the db api) is to use a cross-group transaction to write my own reference on the "one", either by updating an ndb.StringProperty(repeat=True) containing all the related kinds when an entity of a new kind is introduced or by simply maintaining a list of keys on the "one" ndb.KeyProperty(repeat=True) every time a related "many" entity is written to the datastore.

我希望比我更有经验的人可以提出更好的方法.

I'm hoping someone more experienced than myself can suggest a better approach.

鉴于 jmort253 的建议，我将尝试用一个改编自文档的具体示例来补充我的问题:

Given jmort253's suggestion, I'll try to augment my question with a concrete example adapted from the docs:

class Contact(ndb.Expando):
    """ The One """

    # basic info
    name = ndb.StringProperty()
    birth_day = ndb.DateProperty()

    # If I were using db, a collection called 'phone_numbers' would be implicitly 
    # created here.  I could use this property to retrieve related phone numbers 
    # when this entity was queried.  Since NDB lacks this feature, the service 
    # will neither have a reference to query nor the means to know the 
    # relationship exists in the first place since it cannot be hard-coded.  The
    # data model is extensible and user-defined at runtime; most relationships
    # will be described only in the data, and must be discoverable by the server.
    # In this case, when Contact is queried, I need a way to retrieve the
    # collection of phone numbers.

    # Company info.
    company_title = ndb.StringProperty()
    company_name = ndb.StringProperty()
    company_description = ndb.StringProperty()
    company_address = ndb.PostalAddressProperty()

class PhoneNumber(ndb.Expando):
    """ The Many """

    # no collection_name='phone_numbers' equivalent exists for the key property
    contact = ndb.KeyProperty(kind='Contact')
    number = ndb.PhoneNumberProperty()

推荐答案

有趣的问题！所以基本上你想查看 Contact 类并找出是否有其他模型类具有引用它的 KeyProperty；在这个例子中电话号码(但可能有很多).

Interesting question! So basically you want to look at the Contact class and find out if there is some other model class that has a KeyProperty referencing it; in this example PhoneNumber (but there could be many).

我认为解决方案是要求您的用户在创建 PhoneNumber 类时明确添加此链接.

I think the solution is to ask your users to explicitly add this link when the PhoneNumber class is created.

您可以通过为用户提供一个 KeyProperty 的子类来处理此问题，从而使他们更容易做到这一点；例如

You can make this easy for your users by giving them a subclass of KeyProperty that takes care of this; e.g.

class LinkedKeyProperty(ndb.KeyProperty):
    def _fix_up(self, cls, code_name):
        super(LinkedKeyProperty, self)._fix_up(cls, code_name)
        modelclass = ndb.Model._kind_map[self._kind]
        collection_name = '%s_ref_%s_to_%s' % (cls.__name__,
                                               code_name,
                                               modelclass.__name__)
        setattr(modelclass, collection_name, (cls, self))

究竟如何选择集合的名称以及存储的值取决于您；只需在那里放一些东西，让您可以轻松地按照链接返回.该示例将在 Contact 上创建一个新属性:

Exactly how you pick the name for the collection and the value to store there is up to you; just put something there that makes it easy for you to follow the link back. The example would create a new attribute on Contact:

Contact.PhoneNumber_ref_contact_to_Contact == (PhoneNumber, PhoneNumber.contact)

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