Angular 2: Two backend service calls on success of first service

Question

In my Angular 2 app I have backend service as below.

getUserInterests() {
    return this.http.get('http://localhost:8080/test/selections').map((res: Response) => res.json());
}

After calling this service I want to call another service on success of previous one.

2nd service

let params: URLSearchParams = new URLSearchParams();
    params.set('access_token', localStorage.getItem('access_token'));
    return this.http.get('http://localhost:8080/user/selections', { search: params }).map((res: Response) => res.json());

These two services separately return two JSON Arrays. Then I need to do some login with these two arrays.

EDITED

service.ts

getUserInterests() {
    return this.http.get('http://localhost:8080/test/selections').map((res: Response) => res.json());
}

getSavedSelections() {
    let params: URLSearchParams = new URLSearchParams();
    params.set('access_token', localStorage.getItem('access_token'));
    return this.http.get('http://localhost:8080/interest/user/selections', { search: params }).map((res: Response) => res.json());
}

getSelectionList() {
    var obs = this.getUserInterests().flatMap(
        (interests) => {
            return Observable.forkJoin([
                Observable.of(interests),
                this.getSavedSelections()
            ]);
        }
    );
    return obs;
}

Then I am using following in my other ts file to call the service.

export class InterestsComponent {
  private interests;
  private saved_interests;
  constructor(private dataService: DataService) {
    this.dataService.getSelectionList().subscribe(
        (result) => {
            var interests = result[0];
            var selections = result[1];
        }
    );
  }
}

But this give following error on console log.

ORIGINAL EXCEPTION: TypeError: this.dataService.getSelectionList is not a function

Any suggestions are appreciated.

Answer 1

You need to leverage the flatMap operator to call one request after the previous one completed:

this.service.getUserInterests().flatMap(
  (interests) => {
    let params: URLSearchParams = new URLSearchParams();
    params.set('access_token', localStorage.getItem('access_token'));
    return this.http.get('http://localhost:8080/user/selections', {
      search: params
    }).map((res: Response) => res.json());
  }
);

When subscribing to this data flow, you will only receive the result of the last request.

You could use the Observable.forkJoin method to return both. Here is a sample:

var obs = this.service.getUserInterests().flatMap(
  (interests) => {
    return Observable.forkJoin([
      Observable.of(interests),
      this.service.getUserSelections()
    ]);
  }
);

obs.subscribe(
  (result) => {
    var interests = result[0];
    var selections = result[1];
  }
);