Firebase - snapshot based on ids - firebase

Question

I've been looking everywhere for the logic to build a specific request, but I don't completely get the firebase philosophy. This is my data structure:
`users:{
u1_id:{...}
u2_id:{...}
...
},
contacts:{
u1_id:{
contact1_id,
contact2_id,
...
}
}`
My first option would be a request that gets the data regarding the user id, the second one (the actual one) is to store the data I need from the contacts when they're added but I would need to run a big update for each contacts of each user and that doesn't seem to be the right solution. (I've check questions, firebase doc and videos)
The closest solution I found is this (from their youtube):
function getUserContacts(key, cb){
const rootRef = firebase.database().ref()
const contactsRef = rootRef.child('contacts')
const usersRef = rootRef.child('users')
contactsRef.child(key).on('child_added', snap => {
let userRef = usersRef.child(snap.key)
userRef.once('value', cb)
})
}
but it gives the users 1 by 1 and not an object with all my users
Edit: my temporary solution ->
firebase.database().ref('users')
.on('value', snaps => {
firebase.database().ref(`contacts/${currentUser.uid}`)
.on('value', snapshot => {
let userContacts = []
snapshot.forEach(snapc => {
snaps.forEach(snapu => {
let duser = snapu
if(snapc.key == snapu.key)
{
userContacts.push(snapu.val())
}
})
})
dispatch({
type: CONTACTS_FETCH,
payload: userContacts
})
})
})

Answer 1

The question is still a bit unclear but let me present three options:
If we want to populate a list with user_1's contacts, we read in the users contacts node by value. In this case, user_1 has two contacts; user_id2 and user_id3
contacts
user_1
user_id2: true
user_id3: true
Value reads in the parent node (user_1) with all of the child data as well. Because we have the entire node of data (a snapshot) we can iterate over the child values of user_1 to obtain each user_idx (which is the key of each child node). As you iterate over the children, each will present a key, to which you can then read in that specific user node.
A second option is to change the structure.
contacts
user_2
name: "John"
rank: "Captain"
contact_of: "user_1"
user_3
name: "William"
rank: "Second Officer"
contact_of: "user_1"
with this structure, if user 1 wants to load their contacts, a simple query on the contacts node where contact_of = "user_1". That would load each contact and all of their data.
This second example works when a user has specific contacts that are not shared.
If multiple users can have multiple contacts, here's another option.
contacts
user_2
name: "Jean Luc"
rank: "Captain"
contact_of
user_1: true
user_2: true
user_3
name: "William"
rank: "Second Officer"
contact_of:
user_1: true
user_2: true
in this case, perform a deep query on the contacts node where contact_of/user_1 = true, which will present user_2 and user_3/