Retrieving child value of unknown parent Firebase, AngularJS - angularjs

Question

I have basic project on Firebase, AngularJS.
{
"users" : {
"X3IuXTMi9RgOLMKBoqPJtB1JGGQ2" : {
"profile" : {
"firstName" : "test",
"lastName" : "5",
"username" : "test5"
}
},
"poPg2BfHA1gq3qtXNLbuw4X2Zt23" : {
"profile" : {
"firstName" : "test3",
"lastName" : "asdfg",
"username" : "test3"
}
}
}
}
i want to add users by username basically. when type username on alertprompt(ionic) and click add, it will find target username and get uid on its parent. and it will write data on "users/$targetuid/friendrequests/$senderuid"
my closest search result was that: Retrieve data of child of unknown parent ID - Firebase
but i cant use any .ref codes;
Cannot read property 'ref' of undefined
i just can use FirebaseObjectObservable, what im i doing wrong ? why cant i use
firebase.database().ref
or any like this code based on .ref command?
basically cant do anything here: https://firebase.google.com/docs/database/web/read-and-write
im am sort of new on coding, so if its easy or funny dont laugh much. thanks for reading.

Answer 1

I can see two solutions here:
1) Create a usernameToUserid map: on firebase create a new resource usernameToUserid, ike below:
{
"users" : {
"X3IuXTMi9RgOLMKBoqPJtB1JGGQ2" : {
"profile" : {
"firstName" : "test",
"lastName" : "5",
"username" : "test5"
}
},
"poPg2BfHA1gq3qtXNLbuw4X2Zt23" : {
"profile" : {
"firstName" : "test3",
"lastName" : "asdfg",
"username" : "test3"
}
}
},
"usernameToUserid: {
"test5": "X3IuXTMi9RgOLMKBoqPJtB1JGGQ2",
"test3": "poPg2BfHA1gq3qtXNLbuw4X2Zt23"
}
}
This way you can retrieve the userId and then the user profile with the username.
2) Retrieve all users and search locally for username, terrible idea.