users: {
    uid: {
        username: exampleName
        email: exampleEmail
        personalInfo: examplePersonalInfo
    }
}

db.database.ref('users').orderByChild('username').equalTo('exampleName')
    .once('value').then(handler);

personalInfo: {
    ".read" = "$uid === auth.uid"
 }

username: {
     ".read" = true
 }