Listing Firebase data and relationships - ios

Question

I have just started working with Firebase database and I am a bit confused how to structure the database. In the following example I have a users object and a groups object. Each user can be part of multiple groups and every group can have multiple users. The proposed database structure is the following, according to "Structure Your Database".
{
"users": {
"alovelace": {
"name": "Ada Lovelace",
"groups": {
"techpioneers": true,
"womentechmakers": true
}
}
},
"groups": {
"techpioneers": {
"name": "Historical Tech Pioneers",
"startDate": "24-04-1820",
"members": {
"alovelace": true,
"ghopper": true,
"eclarke": true
}
}
}
}
Let's say I want to display all groups in a list in my app, with the group name and start date. How would I make that database call? Since the user object only contains the id of the groups, would I then have to make a separate call to the database for every group just to find out the name and start date of the group? If there are many groups in the list, then that becomes a lot of calls. My group might contain a lot of other information as well so this doesn't seem good for performance. Can I get all the groups in the groups list of the user, in one call?
One though I had was to include the name and start date in the groups object under the user:
"users": {
"alovelace": {
"name": "Ada Lovelace",
"groups": {
"techpioneers":{
"name": "Historical Tech Pioneers",
"startDate": "24-04-1820"
},
"womentechmakers":{
"name": "Women in Technology",
"startDate": "13-10-1823"
}
}
}
}
}
but this solution seems to add a lot of duplicate data. Also if I want to update the name I would have to do that in multiple locations. And maybe I want to add a sponsor organization object, that also contains group, and then want to list them. Then there would be 3 places to update the information on. How would I solve this?

Answer 1

You would then have two possibilities, one would be to store the data you need (duplicating it) in the groups node of each user.
The other, which is the one that I would recommend the most, would be to add an observeSingleEvent(of: .value) inside your first observer (that could be an observe(.value), observe(.childAdded) or whatever).
Say you have an array of all your group members, and an object named AppUser that represents a user in your app :
var groupMembers = [AppUser]()
To detect whenever a new member is added to a group for example, you could use a .childAdded observer for example :
func groupWasAddedObserver(completion: #escaping () -> ()) {
// Add a .childAdded observer to the group's members node (groupId should be defined somewhere) :
groupsRef.child(groupId).child("members").observe(.childAdded, with: { [weak self] (snapshot) in
// When you get the snapshot, store its key, which represents the member id :
let memberId = snapshot.key
// fetch this member's profile information using its id :
self?.getUser(memberId, completion: { (groupMember) in
// Once you got the profile information of this member, add it to an array of all your members for example :
self?.groupMembers.append(groupMember)
// Call the completion handler so that you can update the UI or reload a table view somewhere maybe depending on your needs :
completion()
})
})
}
And the second method to fetch a user data knowing his or her id :
func getUser(_ userId: String, completion: #escaping (AppUser) -> ()) {
// Add the observerSingleEvent observer :
usersRef.child(userId).observeSingleEvent(of: .value, with: { (snapshot) in
// Get the data you need using the snapshot you get :
guard let email = snapshot.childSnapshot(forPath: "email").value as? String else { return }
guard let name = snapshot.childSnapshot(forPath: "name").value as? String else { return }
guard let picUrl = snapshot.childSnapshot(forPath: "picUrl").value as? String else { return }
// Call the completion handler to return your user/member :
completion(AppUser(id: snapshot.key, email: email, name: name, picUrl: picUrl))
})
}
As you can see you get the memberId of each user using the snapshot key, and you use this memberId to fetch this specific user data.