ndb 建模一对多:重复 KeyProperty 与外键的优点 [英] ndb modelling one-to-many: merits of repeated KeyProperty vs foreign key

问题描述

我的问题是关于在 ndb 中建模一对多关系.我知道这可以通过(至少)两种不同的方式完成:使用重复的属性或使用外键".我在下面创建了一个小例子.基本上我们有一个可以有任意数量标签的文章.让我们假设一个标签可以被删除，但在添加后不能更改.我们还假设我们不担心交易安全.

My question is about modelling one-to-many relations in ndb. I understand that this can be done in (at least) two different ways: with a repeated property or with a 'foreign key'. I have created a small example below. Basically we have an Article which can have an arbitrary number of Tags. Let's assume that a Tag can be removed but cannot be changed after it has been added. Let's also assume that we don't worry about transactional safety.

我的问题是:建模这些关系的首选方法是什么?

我的考虑:

• 方法 (A) 需要为每个添加到一个标签的标签写入两次文章(一个用于文章，一个用于标签)而方法(B) 只需要一次写入(仅标签).
• 方法 (A) 利用获取文章的所有标签时 ndb 的缓存机制，而在方法 (B) 的情况下，需要一个查询(另外还有一些自定义缓存)

这里有我遗漏的地方吗?还有其他需要考虑的因素吗?

Are there some things that I'm missing here, any other considerations that should be taken into account?

非常感谢您的帮助.

示例(A):

class Article(ndb.Model):
    title = ndb.StringProperty()
    # some more properties
    tags = ndb.KeyProperty(kind="Tag", repeated=True)

    def create_tag(self):
        # requires two writes
        tag = Tag(name="my_tag")
        tag.put()
        self.tags.append(tag)
        self.put()

    def get_tags(self):
        return ndb.get_multi(self.tags)

class Tag(ndb.Model):
    name = ndb.StringProperty()
    user = ndb.KeyProperty(Kind="User") #  User that created the tag
    # some more properties

示例(B):

class Article(ndb.Model):
    title = ndb.StringProperty()
    # some more properties

    def create_tag(self):
        # requires one write
        tag = Tag(name="my_tag", article=self.key)
        tag.put()

    def get_tags(self):
        # obviously we could cache this query in memcache
        return Tag.gql("WHERE article :1", self.key)

class Tag(ndb.Model):
    name = ndb.StringProperty()
    article = ndb.KeyProperty(kind="Article")
    user = ndb.KeyProperty(Kind="User") #  User that created the tag
    # some more properties

推荐答案

您是否看过以下有关使用 结构化属性 https://developers.google.com/appengine/docs/python/ndb/properties#structured.那里关于 Contact 和 Addresse 的简短讨论可能会简化您的问题.另请参阅 https://developers.google.com/appengine/docs/python/ndb/queries#filtering_structured_properties.讨论很短.

Have you looked at the following about using Structured Properties https://developers.google.com/appengine/docs/python/ndb/properties#structured . The short discussion there about Contact and Addresse may simplify your problem. Also look at https://developers.google.com/appengine/docs/python/ndb/queries#filtering_structured_properties. The discussions are very short.

另外，展望不允许连接的事实，选项 A 看起来更好.

Also, looking ahead to the fact that joins are not allowed, option A looks better.

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