constructor( @Inject(FirebaseRef) fb) {

      const rootDbRef = fb.database().ref();

      rootDebRef.child('courses').on('value', 
          snap => console.log('Received the whole courses node',snap.val()) );
      
      rootDebRef.child('lessons/-KT_udWS6pEmpLVrxlVw:').on('value', 
          snap => console.log('Received lesson with a given Id',snap.val()) );

}