title: Merge results of array of Observables
body: <p>I need to fetch IDs from my state as separate <code>Observable</code>s, combine all of them into one <code>Observable</code> and <code>subscribe()</code> to it.
  I need that my subscriber will eventually get an array of resolved <code>Observable</code> results. </p>
  
  <p>Also I need to keep this subscription open, so if one of my inner <code>Observable</code>s changes my subscriber will be notified.</p>
  
  <p>This is my code:</p>
  
  <pre class="lang-typescript prettyprint-override"><code>getTransactionsByIDs(transactionsIDs){
  return Observable.of(transactionIDs
    .map(transactionID =&gt; this.getTransactionByID(transactionID)));
  }
  
  this.transactionsService.getTransactionsByIDs(transactionsIDs)
  .subscribe(transactions=&gt;{
   ....
  })
  </code></pre>
  
  <p>The result of the above code in the subscriber function is an array of unresolved stores.</p>
  
  <p>How can I resolve each store and combine all of them together?</p>
  
  <p>I've also tried to use <code>Observable.from()</code> on the <code>transactionsID</code>s to transform each ID into an <code>Observable</code> and then resolve it. It works fine, but then my subscriber getting notified on every ID separately. If there's a way to batch all of the <code>Observable.from()</code> results (while keeping subscription open), please let me know.</p>
  
  <p>This is how my <code>Observable.from()</code> looks like:</p>
  
  <pre class="lang-typescript prettyprint-override"><code>getTransactionsByIDs(transactionsIDs){
  return transactionIDs
    .mergeMap(transactionID =&gt; this.getTransactionByID(transactionID));
  }
  
  this.transactionsService.getTransactionsByIDs(Observable.from(transactionsIDs))
  .subscribe(transactions=&gt;{
   ....
  })
  </code></pre>