4. Querying lists Querying is a killer feature of AngularFire2. You can specify query values as observables, and when those observables emit new values, the query is automatically re-run. Creating a query with primitive/scalar values Queries are created by specifying a query object on the FirebaseListObservable options. const queryObservable = af.database.list( '/items' , { query: { orderByChild: