How to dynamically query entities from an akita entity store

I have an entity like this

interface IDevice {
  id: string;
  name: string;
  brand: string;
  plan: 'contract' | 'unlocked';
}

and an entity store

interface DevicesState extends EntityState<IDevice> {
  selectedBrand: string | undefined;
  selectedPlan: 'contract' | 'unlocked';
}

And I want to query and filter the entities based on the brand selected.

My only attempt was this

  selectedBrand$ = this.select('selectedBrand');
  selectedPlan$ = this.select('selectedPlan');
  devices$ = this.selectedBrand$ 
    ? this.selectAll({
        filterBy: [
         entity => entity.brand === this.selectedBrand$,
         entity => entity.plan === this.selectedPlan$,
        ]
      })
    : this.selectMany([]);

This will not work since this.selectedBrand$ is an observable. How do I select the devices based on two outer state values?

You can use switchMap to first observe the selected brand and then switch to the store select based on the selected brand:

selectedBrand$ = this.select('selectedBrand');
devices$ = this.selectedBrand$
  .pipe(
      switchMap(brand => brand 
          ? this.selectAll({
                filterBy: entity => entity.brand === brand
          }) 
          : this.selectMany([])
      )
  );

When you want to combine two observables, you can use combineLatest:

  devices$ = combineLatest(this.selectedBrand$, this.selectedPlan$)
    .pipe(
      switchMap(([brand, plan]) => this.selectAll({
          filterBy: [
            brand ? entity => entity.categories.map(x => x.name).includes(brand) : null,
            plan ? entity => entity.plan_tab === plan : null
          ].filter(x => x !== null)
      })
    );