javascript - Firebase v3 Query by Grandchild

Question

When registering new email/password type users, I need to make user that the displayName that they enter does not already exist in my Realtime Database before calling .createUserWithEmailAndPassword. So I need a query. I need to get a list of all displayName's in my users branch. I am confused as to how to gather this for each user which is denoted by each users auth.uid.

What would the retrieval look like? I am thinking something like:

firebase.database().ref('users/' + allUserIds).equalTo('Joe');

but I know that allUserIds is not valid. Can someone help me with this?

{
  "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"
    }
  }
}

Answer 1

Waitting for answers