Just started using Vaadin and GAEContainer and first of all thanks Johan for this with well commented code 
But as I see in sources, it keeps entity count for each kind in in “_size” store. But it relies on that once it is created in datastore and makes trouble when real entity count is different from this number.
I started my app as JDO, so in other parts i just create and persist entities this way and its not really working together.