title: NDB Model Querying of Key Ids using an array filter
body: <p>I'm trying to query an NDB model using a list of provided key id strings.  The model has string ids that are assigned at creation - for example:</p>
  
  <pre><code>objectKey = MyModel(
      id="123456ABC",
      name="An Object"
      ).put()
  </code></pre>
  
  <p>Now I can't figure out how to query the NDB key ids with a list filter.  Normally you can do the MyModel.property.IN() to query properties:</p>
  
  <pre><code>names = ['An Object', 'Something else', 'etc']
  
  # This query works
  query = MyModel.query(MyModel.name.IN(names))
  </code></pre>
  
  <p>When I try to filter by a list of keys, I can't get it to work:</p>
  
  <pre><code># This simple get works
  object = MyModel.get_by_id("123456ABC")
  
  ids = ["123456ABC", "CBA654321", "etc"]
  
  # These queries DON'T work
  query = MyModel.query(MyModel.id.IN(ids))
  query = MyModel.query(MyModel.key.id.IN(ids))
  query = MyModel.query(MyModel.key.id().IN(ids))
  query = MyModel.query(MyModel._properties['id'].IN(ids))
  query = MyModel.query(getattr(MyModel, 'id').IN(ids))
  ...
  </code></pre>
  
  <p>I always get <code>AttributeError: type object 'MyModel' has no attribute 'id'</code> errors.  </p>
  
  <p>I need to be able to filter by a list of IDs, rather than iterate through each ID in the list (which is sometimes long). How do I do it?</p>