How can I get @ngrx/data to retrieve data only when the entityCache is empty?

Question

I am customising an entity service in @ngrx/data, and I would like to have the store cache the data, such that it only retrieves the data from the API once, when it hasn't loaded yet, but skips any subsequent calls to the API.

So far, I can't seem to get a method override working:

    load(options?: EntityActionOptions | undefined): Observable<Product[]> {
        return this.loaded$.pipe(
            tap(loaded => {
                if (!loaded) {
                    super.load(options);
                }
            }),
            filter(loaded => loaded),
            switchMap(() => this.entities$)
        );
    }