如何将http调用的输出作为输入传递给RxJS中的另一个http调用？

嗨，我尝试执行以下操作时出错

export class FlightFormTemplatesStepComponent
{

 tagonMessagePrefix: TagOnMessagePrefix;


  ngOnInit() {
     this.flightService.get(this.flightId).pipe(
         tap (flight => {this.flight = flight}),
         mergeMap( (flight) => {
           return this.flightService.getTagonMessagePrefix(flight);
         })
        ).subscribe(
          (tagonMessagePrefix) => { this.tagonMessagePrefix = tagonMessagePrefix}

        );
   }
}

这就是我收到的错误

property) FlightFormTemplatesStepComponent.tagonMessagePrefix: TagOnMessagePrefix
Type '{}' is missing the following properties from type 'TagOnMessagePrefix': id, flightName, flightTagonTypes, configuredGlobalTagonSettings, tagonTextts(2739)

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你知道我该如何解决这个问题吗？

我改为映射操作，它可以正常工作

this.flightService.get(this.flightId).pipe(
 tap (flight => {this.flight = flight}),
 switchMap( (flight) => {
   return this.flightService.getTagonMessagePrefix(flight);
 })
).subscribe(
  (tagonMessagePrefix:TagonMessagePrefix) => { this.tagonMessagePrefix = tagonMessagePrefix}

);