title: AdWords API CampaignService.Query() returns INVALID_PREDICATE_FIELD_NAME for valid fields
body: <p>I'm having trouble getting a simple <a href="https://developers.google.com/adwords/api/docs/guides/awql" rel="nofollow">AWQL</a> syntax to work with the AdWords API CampaignService.  I'm trying to gather a list of all CampaignIds that have greater than 10 impressions for a date range:</p>
  
  <pre><code>campaignService = authorizedClient.GetService('CampaignService', version='v201406')
  
  awql = "SELECT CampaignId WHERE Impressions &gt; 10 DURING 20150120,20150223"
  
  results = campaignService.query(awql)
  </code></pre>
  
  <p>The above syntax gives the following error:</p>
  
  <pre><code>Server raised fault: '[SelectorError.INVALID_PREDICATE_FIELD_NAME @ query; trigger:'Impressions'; errorDetails:Impressions] Original AdsAPI trace for debugging [
  com.google.ads.api.services.common.error.ApiException: [SelectorError.INVALID_PREDICATE_FIELD_NAME @ query; trigger:'Impressions'; errorDetails:Impressions]
  </code></pre>
  
  <p>Whenever I adjust the Awql syntax as follows, I get a successful result:</p>
  
  <pre><code>campaignService = authorizedClient.GetService('CampaignService', version='v201406')
  
  # Works
  awql = "SELECT CampaignId"
  results = campaignService.query(awql)
  
  # Works
  awql = "SELECT CampaignId DURING 20150120,20150223"
  results = campaignService.query(awql)
  
  # Doesn't Work
  awql = "SELECT CampaignId WHERE Impressions &gt;10 DURING 20150120,20150223"
  results = campaignService.query(awql)
  </code></pre>
  
  <p>I've tried using <code>impression</code>, <code>Impression</code>, <code>impressions</code> and none of those work either.  I can confirm via the AdWords UI that there are campaigns on this account that have impressions during this time period.</p>
  
  <p>Why won't the CampaignService let me query with Impressions as a <code>WHERE</code> filter?</p>
  
  <p>Edit: I found a list of <a href="https://developers.google.com/adwords/api/docs/reference/v201409/MutateJobService.Campaign" rel="nofollow">filterable fields</a> here for campaigns, but in the documentation example <a href="https://developers.google.com/adwords/api/docs/guides/awql" rel="nofollow">here</a> it shows them using Impressions?</p>
  
  <pre><code>CampaignPage p = campaignService.query("SELECT CampaignId,CampaignName
                  WHERE Ctr &gt; 0.05 AND Impressions &lt; 100
                  DURING 20120201,20120228 ORDER BY CampaignName DESC LIMIT 0,50");
  </code></pre>