title: Mongo query for field name
body: <p>I have a collection that summarizes some data related to documents in other collections. It's structure is roughly like:</p>
  
  <pre><code>{
      campaignId : ObjectId(...)
      impressions : {
          ...
          '2016-09-20': 1800,
          '2016-09-21': 1500,
          '2016-09-22': 2000
      },
      clicks : {
          ...
          '2016-09-20': 60,
          '2016-09-21': 55,
          '2016-09-22': 80
      }
  }
  </code></pre>
  
  <p>I realize there are better ways of doing it, but it's something I can't control.</p>
  
  <p>The issue is that I need to <strong>query for documents that had impressions in the previous 7 days</strong>.</p>
  
  <p>So I need to query based on the field key instead of the value.</p>
  
  <p>Is it possible doing this in a query?</p>