Firebase simple many to many relationship

Question

I am bit familiar with NoSQL and Firebase Realtime Database and also I know that it is not best solution to solve tasks where relational database should be more appropriate. I want to verify about structure of simple many to many relationship that I have.

I have events and users. I want to use Firebase for storing information about users participating in events, later I will need to

Get list of users for event knowing it's id and city
Get list of events for users knowing it's id and city
add or delete information about user attending to event

I would like to have first tree of events ids divided by cities.

events {
'city1' : {
           event_id_1 : {'user_1', 'user_2'},
           event_id_2 : {'user_3', 'user_4'}. 
           }
'city2' : {
           event_id_3 : {'user_5', 'user_6'},
           event_id_4 : {'user_7', 'user_7'}. 
           }
}

And second tree for users

users {
'user1' : {
           'city1' : {event_id_1, event_id_2}, 
           'city2' : {event_id_3, event_id_4},
           'city3' : {event_id_3, event_id_4}
           }, 
'user2' : {
           'city1' : {event_id_1, event_id_2}, 
           'city2' : {event_id_3, event_id_4},
           'city3' : {event_id_3, event_id_4}
           },
'user3' : {
           'city1' : {event_id_1, event_id_2}, 
           'city2' : {event_id_3, event_id_4},
           'city3' : {event_id_3, event_id_4}
           },
}

Would it be easy and fast to use and maintain?

score 1 · Answer 1 · answered Mar 06 '18 at 12:24

You can do this:

List of user

 Users
  useruid 
    name:userx
    email:userx@gmail.com
  useruid
     name:usery
     email:usery@gmail.com

  Events
    eventid
       useruid
         name:userx
         location: city1
         eventname: party
    eventid2
       useruid1
          name:usery
          location: city2
          eventname: Boring Party

Get list of users for event knowing it's id and city:

DatabaseReference ref=FirebaseDatabase.getInstance().getReference().child("Events").child(eventid);
ref.orderByChild("location").equalTo(city1);

//retrieve users using a listener

Get list of events for users knowing it's id and city:

DatabaseReference ref=FirebaseDatabase.getInstance().getReference().child("Events");
Query q=ref.orderByChild("location").equalTo(city1);

using a listener this can give you the events that are in location:city1

Thank you, and how can I check/update that user is going to event? — moonvader, Mar 06 '18 at 12:53
you can add a `response:accepted` or `response: rejected` under the uid of each user — Peter Haddad, Mar 06 '18 at 13:15

score 1 · Accepted Answer · answered Mar 06 '18 at 14:00

Your structure looks pretty OK to me given the requirements listed. Most importantly: you store the data in both directions already, which is the biggest hurdle for many developers new to NoSQL data modeling.

A few notes about your data model, though most are on the level of typos:

Be sure to store the data as maps, not arrays. So event_id_1 : {'user_1': true, 'user_2': true }
If there is a many-to-many relationship between users and events, I'd usually have four top-level lists: users and events (for the primary information about each), and then userEvents and eventUsers (for connections between the two).

Adding a user to an event can be done with a single multi-location update, e.g.:

ref.update({
  '/userEvents/userId1/eventId1': true,
  '/eventUsers/eventId1/userId1': true
});

Unregistering them is a matter of doing the same with null as the value (which deletes the existing key):

ref.update({
  '/userEvents/userId1/eventId1': null,
  '/eventUsers/eventId1/userId1': null
});

Also see my answer here: Many to Many relationship in Firebase

thank you, what is the point in storing true or null instead of just storing ids of entities? — moonvader, Mar 06 '18 at 14:59
The second example *deletes* the keys. The first example stores `true`, since you can't store `null` and [Firebase recommends against using arrays for set-like structures](https://firebase.googleblog.com/2014/04/best-practices-arrays-in-firebase.html) (also see [my answer here](https://stackoverflow.com/questions/40656589/firebase-query-if-child-of-child-contains-a-value)). — Frank van Puffelen, Mar 06 '18 at 17:07

Firebase simple many to many relationship

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