title: Cannot reference firebase data by push id
body: <p>I am trying to update a post in firebase base on its id.
  So I have a variable stored somewhere like this </p>
  
  <pre><code>var postID = -xyz1234567;
  firebase.database().ref('posts').child(postID).update({
    blah: "blah",
    foo: 'bar'
  })
  </code></pre>
  
  <p>but I keep getting this error that says; </p>
  
  <blockquote>
    <p>Error: Firebase.child failed: First argument was an invalid path:
    "undefined". Paths must be non-empty strings and can't contain ".",
    "#", "$", "[", or "]"</p>
  </blockquote>