Firebase - Get data from Service inside Service

Ionic Framework
1

Hello!

I have 2 Services. One for Subjects, one for Teachers. The Data is stored in the Firebase Firestore.
My Subject Object has a property named teacherId. Then i get the Subject objects from Firebase, I want to get the teacher based on its Id stored in subject from the teacherservice. But i do not know how to do this. I tried the following but i doesn’t work:

import { TeacherService } from './../teacher/teacher.service';
import { Subject } from './../../objects/subject';
import { map } from 'rxjs/operators';
import { Injectable } from '@angular/core';

import {AngularFirestore, AngularFirestoreCollection} from 'angularfire2/firestore'
import { Observable } from 'rxjs';






@Injectable({
  providedIn: 'root'
})
export class SubjectService {
  private subjectsCollection : AngularFirestoreCollection<Subject>;
  private subjects :Observable<Subject[]>;
  public subjectsLocal;
  constructor(db : AngularFirestore, private teacherService : TeacherService  ) {
    this.subjectsCollection = db.collection<Subject>('subjects');
     this.subjects = this.subjectsCollection.snapshotChanges().pipe(
        map(actions => {
          return actions.map(a => {
            const id = a.payload.doc.id;
            let data = a.payload.doc.data();    
            this.teacherService.getTeacher("S7XHgaMhpywmezusLrp2").subscribe(res => {
              data.name = res.name;
            });
            return {id, ...data};
          });
        }));
   }

   getSubjects() {
     return this.subjects;
   }

   getSubject(id) {
    return this.subjectsCollection.doc<Subject>(id).valueChanges();
   }

   updateSubject(subject : Subject, id : string) {
      return this.subjectsCollection.doc(id).update(subject);
   }

   addSubject(subject : Subject) {
     this.subjectsCollection.add(subject);
   }

   removeSubject(id) {
     this.subjectsCollection.doc(id).delete();
   }


}

The Teacherservice looks like this:

import { Teacher } from './../../objects/teacher';
import { map } from 'rxjs/operators';
import { Injectable } from '@angular/core';

import {AngularFirestore, AngularFirestoreCollection} from 'angularfire2/firestore'
import { Observable } from 'rxjs';






@Injectable({
  providedIn: 'root'
})
export class TeacherService {
  private teachersCollection : AngularFirestoreCollection<Teacher>;
  private teachers :Observable<Teacher[]>;
  public teachersLocal;
  constructor(db : AngularFirestore  ) {
    this.teachersCollection = db.collection<Teacher>('teachers');

      this.teachers = this.teachersCollection.snapshotChanges().pipe(
        map(actions => {
          return actions.map(a => {
            const id = a.payload.doc.id;
            const data = a.payload.doc.data();
            
            return {id, ...data};
          });
        }));
   }

   getTeachers() {
     return this.teachers;
   }

   getTeacher(id)  {
    return this.teachersCollection.doc<Teacher>(id).valueChanges();
   }

   updateTeacher(teacher : Teacher, id : string) {
      return this.teachersCollection.doc(id).update(teacher);
   }

   addTeacher(teacher : Teacher) {
     this.teachersCollection.add(teacher);
   }

   removeTeacher(id) {
     this.teachersCollection.doc(id).delete();
   }


}

The value of res in subjectservice contains the correct data but it does not apply to the subject.
Is where a solution?
Thanks!

2

Is there a solution?