title: Get data if property exists in firebase
body: <p>So I have a db with 25000 users. And I need to grab the data of the users, which have certain property in their account. Like this:</p>
  
  <pre><code>let users = arrayFromObject(await firebase.database().ref('Users')
      .orderByChild('transactions')
      .once('value').then(r =&gt; r.val()).catch(e =&gt; console.log(e)));
  console.log(users);
  users = users.filter(u =&gt; {
      if (u.transactions) {
          return u;
      }
  });
  console.log(users);
  </code></pre>
  
  <p>And this works. But it turns out, only 630 users have said property. How can only get the users who have the <code>transactions</code> property? I was thinking about something like <code>orderByChild('transactions').exists()</code> but this function doesn't exist as far as I know.</p>
  
  <p>How can I get only the users who have a <code>transactions</code> field in their account, and filter them on the server?</p>