title: Defining a Firestore query by comparing two document's fields values
body: <p>I'm having my first steps with Cloud Firestore, and I am stuck on this one.
  I'm looking for a where query to return me values that are <code>&gt;=</code> the limit.</p>
  
  <pre><code>  [{
      'limit': 100,
      'value': 200 
    }, 
    {
      'limit': 50,
      'value: 50,
    },
    {
      'limit': 95,
      'value': 90, 
    }]
  </code></pre>
  
  <p>I am using the Firebase Admin Python SDK in a Cloud Function.</p>
  
  <p>Any hints?</p>