title: Firestore: Query documents by property of object
body: <p>I've a collection with contacts with a structure like:</p>
  
  <pre><code>name: 'XPTO Company',
  emails: { 
      susan@xpto.com: { name: 'Susan', text: 'manager' },
      fred@xpto.com: { name: 'Fred', text: 'marketing' }
  }
  </code></pre>
  
  <p>How do I retrieve documents with email 'susan@xpto.com'</p>
  
  <p>Something like:</p>
  
  <pre><code>firebase.firestore().collection('contacts')
        .where(new firebase.firestore.FieldPath('emails', email), '==', true).get()
        .then(snap =&gt; {
        })
  </code></pre>