Get Data between two or more tables firebase

Ionic Framework ionic-v3
1

I’m trying get values between tables or more table in firebase but its return the following result

and I don’t know if this way is corret

getUsers() {
    this.service.getMarketers()
        .subscribe( res => {
            this.users = res;
            this.final_data = this.users.map(function (user) {
                return this.service.getUser(user.key)
            })
        });
  }

this way I’m get there in service getUser(id), but its not found service in console I have:
TypeError: Cannot read property ‘service’ of undefined

Or this way, that importing this packages
import { AngularFire, FirebaseListObservable } from ‘angularfire2’;
import * as firebase from ‘firebase/app’;
and

  getUsers() {
    this.service.getMarketers()
        .subscribe( res => {
            this.users = res;
            this.final_data = this.users.map(function (user) {
                return firebase.list('/products/' + `${user.key}`);
                //  return this.af.database.list('/products/' + `${user.key}`);
            })
        });
  }

but return this error
ERROR Error: Uncaught (in promise): Error: Can’t resolve all parameters for HomePage: ([object Object], [object Object], [object Object], ?).
Error: Can’t resolve all parameters for HomePage: ([object Object], [object Object], [object Object], ?).

any idea ?