Retrieve many-to-many relationship data in Firebase

home.ts

/*
* using:
* angular4.0.2
* angularfire2 4.0.0-rc0
* firebase 3.9.0
* rxjs 5.1.1
*/
export class HomePage {

  user: FirebaseObjectObservable<any>;
  groups: Observable<Array<{}>>;
  events: Observable<Array<{}>>;

  constructor(private _auth: AuthService, public afDB: AngularFireDatabase) {

    this.user = this.afDB.object(`/users/${this._auth.uid}`);
    this.user.subscribe(user =>{

      if(user.groups){
        this.groups = Observable.of(user.groups)
        .map(obj => {
          let arr = [];
          Object.keys(obj).forEach((key) =>{
           //get an Observable containing the info for each key in user.groups object.
            arr.push(this.afDB.object(`groups/${key}`)); 
          })
          //zip() all Observables in the array
          let zip = Observable.zip(...arr);

          //return the emitted values (will return an Observable)
          return zip;
        })
        //use switchMap() to flatten the Observables
        .switchMap(val => val)
      }

      if(user.events){
        this.events = Observable.of(user.events)
        .map(obj => {
          let arr = [];
          Object.keys(obj).forEach((key) =>{
            arr.push(this.afDB.object(`events/${key}`)); 
          })
          let zip = Observable.zip(...arr);
          return zip;
        })
        .switchMap(val => val)
      }

    })
  }
}

database example:

{
  "groups": {
    "123": {
      "name": "developers",
      "users": {
        "1": true
      },
      "users_count": 1
    }
  },
  "events": {
    "456": {
      "name": "Developers conference",
      "users": {
        "1": true
      },
      "users_count": 1
    }
  },
  "users": {
    "1": {
      "name": "Jon",
      "groups": {
        "123": true
      },
      "events": {
        "456": true
      }
    }
  }
}