Merging RXJS observable outputs into a single result

Question

Merging RXJS observable outputs into a single result

In my database, I have two nodes set up:

users: {user1: {uid: 'user1', name: "John"}, user2: {uid: 'user2', name: "Mario"}}
homework: {user1: {homeworkAnswer: "Sample answer"}}

Some users may or may not have homework assigned to them.

The goal is to retrieve a complete list of all users along with their respective homework data in a single call and subscription. What approach would be most effective in achieving this?

Below is the expected format for the obtained list using the provided example:

[{uid: 'user1', name: "John", homework: {homeworkAnswer: "Sample answer"}}, {uid: 'user2', name: "Mario"}]

These are the observable references for users and homework:

let usersObservable = this.af.getObservable(`users/`);
let hwObservable = this.af.getObservable(`homework/`);

javascript angular typescript rxjs observable

Answer 1

Answer №1

Essentially, there are two key tasks to accomplish in this scenario, but it's important to pay attention to some specific details.

Retrieve the data from both observables.
Convert the data to achieve your desired outcome.

The initial step can be easily carried out by utilizing the forkJoin method. With forkJoin, you can input multiple observables and it will produce a value as soon as all observables have finished processing. Note: The forkJoin function only emits once all observables have completed their operations. If your data originates from a store or subject, adding a take(1) operator may be necessary to ensure completion.

The conversion process should also be straightforward. Assuming you want to include all users existing in the user object, you can employ Object.keys to iterate through the current user keys and then utilize map for data transformation.

// Necessary Imports:
import { forkJoin } from 'rxjs';
import { take } from 'rxjs/operators';

// Step 1: Utilize fork join to retrieve results from both observables
forkJoin(
    // Adding pipe(take(1)) is essential if observables do not complete.
    usersObservable.pipe(take(1)), 
    hwObservable.pipe(take(1))
)
    // Step 2: Transform the obtained data
    // Both results need to be mapped here. Our goal is to display all
    // users and incorporate any available homework. Using Object.keys, we can iterate over the current user keys within your object.
    .pipe(map(([users, homeworkMap]) => Object.keys(users)
        // Now we can map the keys to the actual user objects
        // and combine them with the corresponding homework
        .map(userKey => {
            const user = users[userKey];
            const homework = homeworkMap[userKey];
            return {
                ...user,
                homework
            };
        })
    ))
    .subscribe(users => console.log('Users: ', users));

Answer 2

Essentially, there are two key tasks to accomplish in this scenario, but it's important to pay attention to some specific details.

Retrieve the data from both observables.
Convert the data to achieve your desired outcome.

The initial step can be easily carried out by utilizing the forkJoin method. With forkJoin, you can input multiple observables and it will produce a value as soon as all observables have finished processing. Note: The forkJoin function only emits once all observables have completed their operations. If your data originates from a store or subject, adding a take(1) operator may be necessary to ensure completion.

The conversion process should also be straightforward. Assuming you want to include all users existing in the user object, you can employ Object.keys to iterate through the current user keys and then utilize map for data transformation.

// Necessary Imports:
import { forkJoin } from 'rxjs';
import { take } from 'rxjs/operators';

// Step 1: Utilize fork join to retrieve results from both observables
forkJoin(
    // Adding pipe(take(1)) is essential if observables do not complete.
    usersObservable.pipe(take(1)), 
    hwObservable.pipe(take(1))
)
    // Step 2: Transform the obtained data
    // Both results need to be mapped here. Our goal is to display all
    // users and incorporate any available homework. Using Object.keys, we can iterate over the current user keys within your object.
    .pipe(map(([users, homeworkMap]) => Object.keys(users)
        // Now we can map the keys to the actual user objects
        // and combine them with the corresponding homework
        .map(userKey => {
            const user = users[userKey];
            const homework = homeworkMap[userKey];
            return {
                ...user,
                homework
            };
        })
    ))
    .subscribe(users => console.log('Users: ', users));

Merging RXJS observable outputs into a single result

Answer №1

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