title: How to get a data from firestore while using batch?
body: <p>I want to perform a batch transaction in firestore. I am storing last key in other collection.
  i need to get the last key then increase by 1, then create two documents using this key. How can i do this?</p>
  
  <pre><code> let lastDealKeyRef = this.db.collection('counters').doc('dealCounter')
   let dealsRef = this.db.collection('deals').doc(id)
   let lastDealKey = batch.get(lastDealKeyRef) // here is the problem..
   batch.set(dealsRef, dealData)
   let contentRef = this.db.collection('contents').doc('deal' + id)
   batch.set(contentRef, {'html': '&lt;p&gt;Hello World&lt;/p&gt;' + lastDealKey })
   batch.commit().then(function () {
   console.log('done') })
  </code></pre>