afire.service.ts

import { Injectable } from '@angular/core';
import {AngularFirestore, DocumentChangeAction} from 'angularfire2/firestore';

@Injectable({
  providedIn: 'root'
})
export class AfireService {

  constructor(
    private afs: AngularFirestore,
  ) {}

  paginate (limit: number, last: string):  Observable<DocumentChangeAction<any>[]> {
     return this.afs.collection('todos', (ref) => (
      ref
        .where('id', '<', last)
        .orderBy('id', 'desc')
        .limit(limit)
     )).snapshotChanges();
  }
}