title: I can't query GQL for 2 properties only wildcard is working
body: <p>I store <code>comment</code> entity in Datastore and I use <code>Datastore Viewer</code> to see stored data.</p>
  
  <p>I can do following queries</p>
  
  <pre><code>select * from comment    
  select __key__ from comment
  select Subject from comment
  select Comment from comment
  </code></pre>
  
  <p>But I can't query</p>
  
  <pre><code>select Subject, Comment from comment
  // Error: The suggested index for this query is:
  //    - kind: comment
  //      properties:
  //      - name: Comment
  //      - name: Subject
  select __key__, Subject, Comment from comment
  // Error: projections are not supported for the property: __key__
  </code></pre>
  
  <p>I can't find any reference why it is wrong. These errors does not tell me much. </p>
  
  <p>I have not set any key or index for these entities.</p>
  
  <p><a href="https://developers.google.com/appengine/docs/python/datastore/gqlreference" rel="nofollow">Documentation</a> tells following:</p>
  
  <pre><code>SELECT [DISTINCT] [* | &lt;property list&gt; | __key__]
    [FROM &lt;kind&gt;]
    [WHERE &lt;condition&gt; [AND &lt;condition&gt; ...]]
    [ORDER BY &lt;property&gt; [ASC | DESC] [, &lt;property&gt; [ASC | DESC] ...]]
    [LIMIT [&lt;offset&gt;,]&lt;count&gt;]
    [OFFSET &lt;offset&gt;]
  
    &lt;property list&gt; := &lt;property&gt; [, &lt;property&gt; ...]
    &lt;condition&gt; := &lt;property&gt; {&lt; | &lt;= | &gt; | &gt;= | = | != } &lt;value&gt;
    &lt;condition&gt; := &lt;property&gt; IN &lt;list&gt;
    &lt;condition&gt; := ANCESTOR IS &lt;entity or key&gt;
    &lt;list&gt; := (&lt;value&gt; [, &lt;value&gt; ...]])
  </code></pre>
  
  <p><code>Subject, Comment</code> - is property list and it valid query. But it is not.
  I created entity from Go code.</p>