Tips for merging an Observable containing a series of values with an Observable containing a single value and subsequently simplifying this arrangement

Question

Tips for merging an Observable containing a series of values with an Observable containing a single value and subsequently simplifying this arrangement

I am searching for the most efficient method to combine two functions that return Observables.

The initial function is described as:

abstract getProfiles(): Observable<ClientProfile[]>

The other one is:

abstract getUser(userId: number): Observable<User>

My goal is to create a data structure projection like this:

interface Projection {
  user: User,
  profiles: ClientProfile[]
}

To achieve this, I am looking to optimize the following function:

getUserProfiles() {
  this.userService.getProfiles()
    .pipe(
      flatMap((ps: ClientProfile[]) => {
         ps.map(p => this.userService.getUser(p.userId))
         // I NEED TO RETURN AN OBSERVABLE HERE:
         // {user: u, ...ps}
      })
    )
    .subscribe(_ => doSomething(_))

What is the most effective way to merge a single user with the list of profiles?

typescript rxjs

Answer 1

Answer №1

To improve efficiency, consider using a combination of the mergeMap and forkJoin operators.

import {map as rxMap, mergeMap} from 'rxjs/operators';
import {forkJoin} from 'rxjs';

this.userService.getProfiles().pipe(
    mergeMap(profiles =>
        forkJoin(profiles.map(profile =>
            this.userService.getUser(profile.userId).pipe(
                rxMap(user =>
                    ({
                        user,
                        profiles
                    })
                )
            )
        )
    )
)

The profiles list is transformed into an

Array<Observable<{user, profile}>>

. These observables complete when the http request returns from the userService.

ForkJoin waits for these observables to finish before emitting an observable that contains the completed values. MergeMap combines these inner observables created by getUser() with the outer observable from getProfiles().

Answer 2

To improve efficiency, consider using a combination of the mergeMap and forkJoin operators.

import {map as rxMap, mergeMap} from 'rxjs/operators';
import {forkJoin} from 'rxjs';

this.userService.getProfiles().pipe(
    mergeMap(profiles =>
        forkJoin(profiles.map(profile =>
            this.userService.getUser(profile.userId).pipe(
                rxMap(user =>
                    ({
                        user,
                        profiles
                    })
                )
            )
        )
    )
)

The profiles list is transformed into an

Array<Observable<{user, profile}>>

. These observables complete when the http request returns from the userService.

ForkJoin waits for these observables to finish before emitting an observable that contains the completed values. MergeMap combines these inner observables created by getUser() with the outer observable from getProfiles().

Answer 3

Answer №2

One approach is to use switchMap to generate an array of Observables from the getUser() API calls:

getUserProfiles() {
    this.userService.getProfiles()
      .pipe(
        switchMap((profiles: ClientProfile[]) => {
           const observables = profiles.map(profile => {
              return this.userService.getUser(profile.userId)
                         .pipe(
                           map(user => ({user, profile: profiles}))
                         );
           });

           return observables;
           // Here you should return an Observable:
           // {user: u, ...profiles}
        }),
        switchMap(response => response)
      )
      .subscribe(data => processUserData(data))

This solution is suitable if you prefer processing each emitted getUser() API response without waiting for all requests to be completed.

If you need to wait for all getUser() API calls to finish before further processing, you can utilize forkJoin() as shown below:

getUserProfiles() {
    this.userService.getProfiles()
      .pipe(
        switchMap((profiles: ClientProfile[]) => {
           const observables = profiles.map(profile => {
              return this.userService.getUser(profile.userId)
                         .pipe(
                           map(user => ({user, profile: profiles}))
                         );
           });

           return forkJoin(observables);
           // Here you should return an Observable:
           // {user: u, ...profiles}
        }),
        tap(result => //result will be an array of {user, profile})
      )
      .subscribe(data => processUserData(data))

I hope this explanation proves helpful.

Answer 4

One approach is to use switchMap to generate an array of Observables from the getUser() API calls:

getUserProfiles() {
    this.userService.getProfiles()
      .pipe(
        switchMap((profiles: ClientProfile[]) => {
           const observables = profiles.map(profile => {
              return this.userService.getUser(profile.userId)
                         .pipe(
                           map(user => ({user, profile: profiles}))
                         );
           });

           return observables;
           // Here you should return an Observable:
           // {user: u, ...profiles}
        }),
        switchMap(response => response)
      )
      .subscribe(data => processUserData(data))

This solution is suitable if you prefer processing each emitted getUser() API response without waiting for all requests to be completed.

If you need to wait for all getUser() API calls to finish before further processing, you can utilize forkJoin() as shown below:

getUserProfiles() {
    this.userService.getProfiles()
      .pipe(
        switchMap((profiles: ClientProfile[]) => {
           const observables = profiles.map(profile => {
              return this.userService.getUser(profile.userId)
                         .pipe(
                           map(user => ({user, profile: profiles}))
                         );
           });

           return forkJoin(observables);
           // Here you should return an Observable:
           // {user: u, ...profiles}
        }),
        tap(result => //result will be an array of {user, profile})
      )
      .subscribe(data => processUserData(data))

I hope this explanation proves helpful.

Answer 5

Answer №3

Feel free to use this solution:

function solve() {
    interface ProfileInfo {
        id: number, name: string, userId: number
    }
    interface UserInfo {
        id: number, username: string
    }

    const fetchProfiles = () => ajax('api/profiles').pipe(
        pluck<AjaxResponse, Array<ProfileInfo>>('response')
    );

    const fetchUser = (id: number) => ajax(`api/users/${id}`).pipe(
        pluck<AjaxResponse, UserInfo>('response')
    );

    fetchProfiles().pipe(
        concatMap(profiles => from(profiles).pipe(map(p => [p, profiles]))),
        mergeMap(([p, profiles]: [ProfileInfo, ProfileInfo[]]) => 
                        fetchUser(p.userId).pipe(map(u => [u, profiles])))
    ).subscribe(console.log);
}

Answer 6

Feel free to use this solution:

function solve() {
    interface ProfileInfo {
        id: number, name: string, userId: number
    }
    interface UserInfo {
        id: number, username: string
    }

    const fetchProfiles = () => ajax('api/profiles').pipe(
        pluck<AjaxResponse, Array<ProfileInfo>>('response')
    );

    const fetchUser = (id: number) => ajax(`api/users/${id}`).pipe(
        pluck<AjaxResponse, UserInfo>('response')
    );

    fetchProfiles().pipe(
        concatMap(profiles => from(profiles).pipe(map(p => [p, profiles]))),
        mergeMap(([p, profiles]: [ProfileInfo, ProfileInfo[]]) => 
                        fetchUser(p.userId).pipe(map(u => [u, profiles])))
    ).subscribe(console.log);
}

Tips for merging an Observable containing a series of values with an Observable containing a single value and subsequently simplifying this arrangement

Answer №1

Answer №2

Answer №3

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