// Wrap some fake data in observables to simulate Firebase results.

const user$ = Rx.Observable.of([

  { key: 'key1', name: 'user1', trainingIds: ['t_key1', 't_key3'] },

  { key: 'key2', name: 'user2', trainingIds: ['t_key2'] }

]);

const training$ = Rx.Observable.from([

  { key: 't_key1', name: 'Training 1', desc: 'Desc 1' },

  { key: 't_key2', name: 'Training 2', desc: 'Desc 2' },

  { key: 't_key3', name: 'Training 3', desc: 'Desc 3' }

]);

const getTraining = (key) => {

  return training$.filter(training => training.key == key);

}

​

const visibleUser$ = user$

  // Flatten the array of users to emit users individually.

  .mergeMap(val => val)

  // Project each user to an observable to get its trainings.

  .mergeMap(user => {

    // Array of observables fetching the trainings of the current user.

    const trainingsArr = user.trainingIds.map(trainingId => getTraining(trainingId));

    // Unwrap the trainings and merge them with the current user.

    return Rx.Observable.from(trainingsArr)

      .mergeAll()

      .toArray()

      .map(userTrainings => Object.assign(user, {trainings: userTrainings}));

  })

  .toArray()

​

visibleUser$.subscribe(val => console.log(val));

​