view-purchase-contract.component.ts

this.selectedPurchaseContract$ = this.store$.pipe(
  select(fromStore.getSelectedProduct),
  filter(product => !!product),
  tap(product => this.store$.dispatch(PurchaseContractActions.loadPurchaseContract({ address: product.contractAddress }))),
  // we switch from one observable to another 
  switchMap(() => this.store$.select(fromStore.getSelectedPurchaseContract)),
  filter(contract => !!contract),
);