getRecords(searchText: BehaviorSubject<string>): Observable<any> {
    return searchText.pipe(
      switchMap(st => {

        const fNameRef = this.afs.collection('students', ref => ref.where('fname', '==', st));
        const lNameRef = this.afs.collection('students', ref => ref.where('lname', '==', st));
        const gradeRef = this.afs.collection('students', ref => ref.where('grade', '==', st));

        const combined = combineLatest([
        fNameRef.snapshotChanges(),
        lNameRef.snapshotChanges(),
        gradeRef.snapshotChanges()]
        );

        return combined.pipe(
          switchMap(docs => {
          const [fNameDocs, lNameDocs, gradeDocs] = docs;
          const c = fNameDocs.concat(lNameDocs, gradeDocs);
          return of(c);
          }),
          debounceTime(250),
          tap(arr => console.log(`${arr.length} reads on students collection`)),
          map(changes => {
          return changes.map(c => {
            const data = c.payload.doc.data();
            const id = c.payload.doc.id;
            return { id, ...data };
            });
          })
        );
      }));
  }