Angular2 subscribe inside subscribe

In Angular2, what would be the correct way to implement a second API call based on the result from the first API call? One of my Angular2 components has the following method. I tried with having a subscribe inside another subscribe on complete and the response from the second subscribe is always 'undefined'.

Edit based on suggestion from CozyAzure.

export interface Result {
  host: string;
  resourceUri: string;
  groupId?: string;
  resource?: any;
}

private curateResults(searchTerm: string, searchResults: SearchResults): Result[] {
    const results: Result[] = [];
    if (searchResults.results !== undefined && searchResults.results.length > 0) {
      searchResults.results.forEach((result: any) => {
        const processedSearchResult: Result = {
          host: result.origin.toString(),
          resourceUri: result.url.toString()
        };
        processedSearchResult.resource = undefined;
        processedSearchResult.groupId = undefined;
        this.bopsHttpService.getResourceData(processedSearchResult.host, processedSearchResult.resourceUri)
          .flatMap((resource: any) => {
            processedSearchResult.groupId = this.getGroupIdFromResource(resource, 'groupId');
            if (processedSearchResult.groupId === undefined) {
              const uriParts = processedSearchResult.resourceUri.split('/');
              const predicate = uriParts[uriParts.length - 2];
              if (predicate === 'group') {
                processedSearchResult.groupId = uriParts[uriParts.length - 1];
                return Observable.empty();
              } else {
                return this.bopsHttpService.getGroupRelation(processedSearchResult.resourceUri.split('/').pop());
              }
            } else {
              return Observable.empty();
            }
          })
          .subscribe((relation: any) => {
            if (relation !== undefined) {
              processedSearchResult.groupId = relation.objectId;
              console.log('Fetched Group ID: ', processedSearchResult.groupId);
            }
          });
        results.push(processedSearchResult);
      });
    }
  return results;
}

My http calls are as follows:

  public getGroupRelation(subjectId: string): Observable<Relation> {
    const path = `${this.bopsServiceUrl}/relation/${subjectId}/group`;
    const queryParameters = new URLSearchParams();
    queryParameters.set('at', new Date().toISOString());
    const options = new RequestOptions({
      headers: this.headers,
      search: queryParameters
    });
    return this.http.get(path, options)
      .map((response: Response) => response.json())
      .catch((error: any) => Observable.throw(error.json().error
        || 'BOPS Server error during get group relation Operation', 
          error.json()));
  }

  public getResourceData(host: string, resourceUri: string): Observable<any> {
    const path = host + resourceUri;
    const queryParameters = new URLSearchParams();
    queryParameters.set('at', new Date().toISOString());
    const options = new RequestOptions({
      headers: this.headers,
      search: queryParameters
    });
    return this.http.get(path, options)
      .map((response: Response) => response.json())
      .catch((error: any) => Observable.throw(error
        || 'BOPS Server error during Get Resource Data Operation'));
  }