Loop array and return data for each id in Observable

This is quite possible using nested mergeMap and map. A key approach to stick with Observable is expanding the contacts array into an observable using from function. Then you collect the data using toArray operator. Providing documented example:

public getCombinedData(): Observable<any> {
  return this.getMultipleRelationData()
      .pipe(
          mergeMap((result: any) =>

              // `from` emits each contact separately 
              from(result.contact).pipe(
                  // load each contact
                  mergeMap(
                      contact => this.getSignleData(contact._id),
                      // in result selector, connect fetched detail 
                      (original, detail) => ({...original, relationship: detail})
                  ),
                  // collect all contacts into an array
                  toArray(),
                  // add the newly fetched data to original result
                  map(contact => ({ ...result, contact})),
              )
          ),
      );
}

---

Edit: result selector is deprecated

In the inner mergeMap, you can use a map afterwards.

public getCombinedData(): Observable<any> {
  return this.getMultipleRelationData().pipe(
      mergeMap((result: any) =>
          // `from` emits each contact separately
          from(result.contact).pipe(
              // load each contact
              mergeMap(
                  contact =>
                      this.getSignleData(contact._id).pipe(
                          map(detail => ({ ...contact, relationship: detail })),
                      ),
                  // collect all contacts into an array
                  toArray(),
                  // add the newly fetched data to original result
                  map(contact => ({ ...result, contact })),
              ),
          ),
      ),
  );
}

Although kvetis's answer can work for you here is what I implemented, just posting it because I found it interesting to solve.

  public getCombinedData(): Observable<any> {
    return this.getMultipleRelationData()
      .pipe(
        mergeMap((result: any) => {
          let allIds = result.contact.map(id => this.getSingleData(id._id));
          return forkJoin(...allIds).pipe(
            map((idDataArray) => {
              result.contact.forEach((eachContact, index) => {
                eachContact.relationship = idDataArray[index];
              })
              return result;
            })
          )
        })
      );
  }

Here is an example solution for your question, I have made dummy Observables. https://stackblitz.com/edit/angular-tonn5q?file=src%2Fapp%2Fapi-calls.service.ts