在NDB中从ComputedProperty函数返回列表的解决方法 [英] Workaround to return a list from a ComputedProperty function in NDB

问题描述

我正在将我的应用转换为使用NDB。我以前有过这样的东西：

I am converting my app to use NDB. I used to have something like this before:

@db.ComputedProperty
    def someComputedProperty(self, indexed=False):
      if not self.someCondition:
          return []
      src = self.someReferenceProperty
      list =  src.list1 + src.list2 + src.list3 + src.list4 \
              + [src.str1, src.str2]
      return map(lambda x:'' if not x else x.lower(), list) 

正如您所看到的，我的生成列表的方法有点复杂，我更喜欢用这种方式来保存它。但是当我开始转换为NDB时，我只是用 @ model.ComputedProperty 取代了 @ db.ComputedProperty ，但后来我得到了这个错误：

As you can see, my method of generating the list is a bit complicated, I prefer to keep it this way. But when I started converting to NDB, I just replaced @db.ComputedProperty by @model.ComputedProperty but then I got this error:

NotImplementedError: Property someComputedProperty does not support <type 'list'> types.

我可以在 model.py 中看到ext.ndb ComputedProperty 继承于 GenericProperty 其中 _db_set_value 除了没有处理 lists

I could see in model.py in ext.ndb that ComputedProperty inherits from GenericProperty where in the _db_set_value there are several if/else statements that handle value according to its type, except that there's no handling for lists

目前它经历了第一个条件，并在返回空列表时发出错误。

Currently it goes through the first condition and gives out that error when I return an empty list.

是否有解决此问题并避免错误的方法？

Is there a way to work around this and avoid the error?

推荐答案

这个完整的功能可以在一个函数内完成，所以它不需要是 ComputedProperty 。仅当您想要执行可能查询的计算时才使用Computed Properties。 ComputedProperty 可以将其索引的标志设置为 False ，但是那么这意味着你不会查询它，因此并不需要将它作为属性。

This whole functionality can be done within a function, so it doesn't need to be a ComputedProperty. Use Computed Properties only when you want to do a computation that you might query for. A ComputedProperty can have its indexed flag set to False but then this means you won't be querying for it, and therefore don't really need to have it as a property.

def someComputedProperty(self):
  if not self.someCondition:
      return []
  src = self.someReferenceProperty
  list =  src.list1 + src.list2 + src.list3 + src.list4 \
          + [src.str1, src.str2]
  return map(lambda x:'' if not x else x.lower(), list) 

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