How to query for references using angularfire?

Question

plans is a root collection with 2 fields: date and recipe. recipe is a reference to a different root collection called recipes. I'm trying to construct an observable chain which emits recipes referenced by plans for the specified date range.

lookup(range: MealPlanRange): Observable<Recipe[]> {
    return this.db.collection('plans', ref=>ref
    .where('date', ">=", range.startDate )
    .where('date', "<=", range.endDate )
    ).valueChanges().pipe(
      // at this point, i have the plans i want, 
      //  but i don't know how to get the recipes
      switchMap(ps=>(/*how to get observable of recipes?*/)),
    );
  }

i tried this.db.doc(p[0].recipe), but that doesn't return an observable. I looked at creating a query which specified multiple ids, but that doesn't seem possible. Any suggestions?

Answer 1

Found it:

lookup(range: MealPlanRange): Observable<Meal[]> {
    return this.db.collection('plans', ref => ref
      .where('date', ">=", range.startDate)
      .where('date', "<=", range.endDate)
    ).valueChanges().pipe(
      switchMap(ps => {
        // return empty array when no plans
        if(ps.length === 0) {
           return of([])
        }
        // for each plan, do a doc lookup and use valueChanges() to get observable of changes
        const recipes = ps.map(p => this.db.doc(p.recipe).valueChanges());
        // use combineLatest to get back into an array of recipes, 
        // any change to any recipe will re-emit
        return combineLatest(...recipes);
      }),
    ) as any;
  }