typescript - RxJS - 将 promise 链转换为 observable 不起作用

我正在尝试转换 Promise 链式调用：

this.tradeTypesLogicService.getList().$promise
    .then(this.filterTradeTypes)
    .then(this.setSelectedTradeType)

到一个 RxJS 可观察对象：

this.tradeTypesLogicService
    .getList() //returns observable now
    .pipe(mergeMap(this.filterTradeTypes), map(this.setSelectedTradeType))
    .subscribe();



protected filterTradeTypes = (tradeTypes: Array<any>) => {
    this.tradeTypes = filter(tradeTypes, (trade) => {
        if (trade.id === this.mappedItem.tradeTypeId && !trade.isActive) {
            trade.label = trade.label + " (inactive)";
            return true;
        }
        return trade.isActive;
    });
    return this.tradeTypes;
}



public setSelectedTradeType = (tradeTypes): void => {
    this.selectedTradeTypeModel = tradeTypes.find(tt => tt.id === this.mappedItem.tradeTypeId);
}

我需要将从 filterTradeTypes 返回的数组传递给 setSelectedTradeType，但是它只传递了该数组中的一项。