Une adaptation de https://medium.com/@simonb90/appels-api-it%C3%A9ratifs-avec-rxjs-a1c2593c558b

recursive-rxjs-with-scan.ts

/**
 * Fetches and constructs the breadcrumb of modules, starting from the last opened on the stack (startModuleId),
 * tracing back to the root module, thanks to each module's parentId.
 *
 * @param startModuleId The last module you want to be opened in the breadcrumb.
 */
private fetchModules(startModuleId: AuditModule['_id']): Observable<AuditModule[]> {
    //=> Each time we need to fetch a new module, we emit the wanted module id on this subject
    const fetchIdSubject = new BehaviorSubject(startModuleId);

    return fetchIdSubject.pipe(
        //=> Fetch each module corresponding to this id
        mergeMap(id => this.auditModuleQuery(id).valueChanges.pipe(
            //=> Wait for data and map on it..
            filter(({ data }) => !!data),
            map(({ data }) => data.auditModule),
            //=> If that's not the root module yet, emit the parentId on the source subject, so it's loaded next
            tap(module => module.parentId && fetchIdSubject.next(module.parentId)))),
        //=> Update the breadcrumb state
        scan(updateBreadcrumb, []),
        //=> Begin by emitting an empty breadcrumb before the first module is loaded
        startWith([]));

    function updateBreadcrumb(breadcrumb: AuditModule[], module: AuditModule): AuditModule[] {
        breadcrumb = [...breadcrumb];

        const currentIndex = breadcrumb.findIndex(candidate => candidate._id == module._id);
        const childIndex = breadcrumb.findIndex(candidate => candidate.parentId == candidate._id);

        const replace = currentIndex >= 0;

        breadcrumb.splice(replace ? currentIndex : childIndex, replace ? 1 : 0, module);

        return breadcrumb;
    }
}