在 App Engine 中使用后写计数器或分片计数器提交计数时，为什么找不到模型实例？
我正在开发 App Engine 应用程序的计数器，并遵循此处概述的示例 http://blog.notdot.net/2010/04/High-concurrency-counters-without-sharding
我无法理解以下几行的需要：
counter = cls.get_by_key_name(name)
if not counter:
counter = cls()
似乎请求了增量为实例，然后当将该增量应用于数据存储时，找不到该实例并被替换为新实例。
分片计数器示例中使用了相同的模式 code.google.com/appengine/articles/sharding_counters.html
counter = SimpleCounterShard.get_by_key_name(shard_name)
if counter is None:
counter = SimpleCounterShard(key_name=shard_name)
在这种情况下，未找到的 key_name 与创建的 key_name 是一致的，因此当 counter. put() 最终被调用，似乎新的会覆盖未找到的。
我的用法与此示例类似，它基于上面的第一个示例： https: //github.com/pamelafox/ragetube/blob/master/models.py
我正在阅读第 66 行，因为“已观看了一首歌曲，是时候更新其观看次数了。获取歌曲，但如果你不能，创建一首新歌并给它这个计数”。那是不对的。
为什么 get_by_key_name() 无法返回记录？是否有一些默认行为使第一个示例起作用（即应该提供某种 key_name ）？如果找不到实例，我只是从函数返回，我可能会遇到什么情况？
I'm working on a counter for an App Engine application and am following the example outlined here http://blog.notdot.net/2010/04/High-concurrency-counters-without-sharding
I'm having trouble understanding the need for the following lines:
counter = cls.get_by_key_name(name)
if not counter:
counter = cls()
It seems an increment was requested for the instance, and then when that increment was to be applied to the datastore, the instance couldn't be found and was replaced with a new one.
The same pattern is used on the Sharding Counters example code.google.com/appengine/articles/sharding_counters.html
counter = SimpleCounterShard.get_by_key_name(shard_name)
if counter is None:
counter = SimpleCounterShard(key_name=shard_name)
In this case, the key_name is consistent between the one that wasn't found and the one that is created, so when counter.put() is eventually called it seems the new one will overwrite the not-found one.
My usage is similar to this example, which is based off the first example above: https://github.com/pamelafox/ragetube/blob/master/models.py
I'm reading line 66 as "a song was viewed and it's time to update its viewcount. Fetch the song, but if you can't, create a new song and give it this count". That can't be right.
Why would get_by_key_name() fail to return a record? Is there some default behavior that makes the first example work (ie, should a key_name of some kind be supplied)? What situation might I find myself in if I simply returned from the function if the instance couldn't be found?
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第一个应该是
The first one should be