如何使用远程API重置数据存储模型的属性类型

我在现有的数据存储中有一个模型。看起来是这样的:

class SomeKind(db.Model):
  name = db.StringProperty(required=True)
  someField = db.BlobProperty(required=True)

在数据存储中大约有20000多个这样的实体 现在我想重新构造这个类型,并使之成为这样:

class SomeKind(db.Model):
  name = db.StringProperty(required=True)
  someField = db.StringProperty(required=True)

1. 在数据存储中循环以删除现有的“someField”数据。从models.py
2. 删除属性
3. 将具有新定义的属性添加到models.py

我在(1)中遇到了问题,我想用远程API删除现有属性:

import sys, time, urllib2

sys.path.append("gae/paths")
...
sys.path.append("myapp/path")
from google.appengine.ext import db
from google.appengine.api import memcache
from google.appengine.ext.remote_api import remote_api_stub
from models import *

def tryPut(db, set, tryLimit=10, seconds=5, trying=1):
  try:
    db.put(set)
    return True
  except urllib2.HTTPError:
    if trying <= tryLimit:
      print "retry (%d of %d) in 5 seconds" % (trying, tryLimit)
      time.sleep(5)
      tryPut(db, set, seconds, trying+1)
    else:
      print urllib2.HTTPError
      sys.exit()

def main():

    remote_api_stub.ConfigureRemoteDatastore(None, 
      '/remote_api', my_auth_func, 'myapp.appspot.com')
    q = db.GqlQuery("SELECT * FROM SomeKind")
    last_cursor = memcache.get('SomeKind/update')

    if last_cursor:
        q.with_cursor(last_cursor)

    set = q.fetch(100)

    while len(set) != 0:
      for someKind in set:
        print someKind.name

        # this doesn't work
        delattr(someKind, "someField")

        # this doesn't work either
        del someKind.someField

      print "update to Google"
      if tryPut(db, set):
        cursor = q.cursor()
        memcache.set('SomeKind/update', cursor)

if __name__ == "__main__":
  main()

我在我的机器上运行这个。

Traceback (most recent call last):
File "./query.py", line 91, in <module>
  main()
File "./query.py", line 66, in main
  del someKind.someField
AttributeError: __delete__

文件( http://code.google.com/intl/en/appengine/articles/update_schema.html )比如说“使用delattr删除过时的属性,然后保存实体”。但从来没有这样的例子。

我的步子对吗? 如何删除该属性?