var ref = firebase.database().ref("users_data/userId/tasks");
ref.orderByChild("solved").equalTo(0).limitToFirst(1)...
users_data
userID
tasks
taskKey1
taskID
taskName
solved
...
taskKey2
taskID
taskName
solved
...
userID
tasks
taskKey1
taskID
taskName
solved
...
taskKey2
taskID
taskName
solved
...
....
Domains(id, name, user_id)
Categories(id, name)
Articles(id, title, ..., user_id, domain_id, category_id)
Categories(id, name, domain_id)