title: Firebase v3 Query by Grandchild
body: <p>When registering new email/password type users,  I need to make user that the <code>displayName</code> that they enter does not already exist in my Realtime Database before calling <code>.createUserWithEmailAndPassword</code>.  So I need a query.  I need to get a list of all <code>displayName</code>'s in my <code>users</code> branch.  I am confused as to how to gather this for each <code>user</code> which is denoted by each users auth.uid.</p>
  
  <p>What would the retrieval look like?  I am thinking something like:</p>
  
  <p><code>firebase.database().ref('users/' + allUserIds).equalTo('Joe');</code></p>
  
  <p>but I know that <code>allUserIds</code> is not valid.  Can someone help me with this?</p>
  
  <pre><code>{
    "users" : {
      "6mJb9vtpbDelyyjirKEf6sSEj8h1" : {
        "name" : "asdfs@asdf.com",
        "provider" : "password",
        "displayName" : "Joe"
      },
      "T7D7qEOPPHXjKSzglKheGJDQNkE3" : {
        "name" : "gfdsdf@hlkjh.com",
        "provider" : "password",
        "displayName" : "Jane"
      },
      "kTG9R0V4aXYsogQfgF633KFYtzV2" : {
        "name" : "Andre3000",
        "provider" : "google.com",
        "displayName" : "Andre"
      }
    }
  }
  </code></pre>