[AngularFire 2] Joins in Firebase

Lets see how to query Firebase.

First thing, when we do query, 'index' will always help, for both SQL and NoSQL. 

In Firebase, we can also set index on props, for example we want to set an 'index' on 'courses' --> 'url' prop, we will use 'url' to locate course object.

How to set up index?

In Firebase console --> database --> Rules:

We tell Firebase, for 'courses' document, we want to set index on 'url'.

After setting up index, then how can we do query?

  findCourseByUrl(courseUrl): Observable<Course>{
    return this.angularFire.database.list('courses', {
      query: {
        orderByChild: 'url',
        equalTo: courseUrl
      }
    })
    .map((courses) => courses[0]); // get courses document which url = courseUrl
  }

We need to pass 'query' object for searching course. Notice we are using 'orderByChild' and 'equalTo'.

Get courses' lessons:

  findAllCourseLessons(courseUrl){
    const course$ = this.findCourseByUrl(courseUrl);

    const lessonsPreCourse$ = course$
      .filter(course => !!course)
      .switchMap((course) => {
        console.log(course);
        return this.db.list(`lessonsPerCourse/${course.$key}`)
      });

    return lessonsPreCourse$
      .map((lessonKeys) => lessonKeys
          .map( (lessonKey) => {
             return this.db.object(`lessons/${lessonKey.$key}`)
          }))
      .flatMap((res) => {
        return Observable.combineLatest(res);
      });

  }

We have a document 'lessonsPreCourse' to maintain the lessons which course includes:

This is normalize the dataset, to avoid nest array.

After getting 'lessonsPreCourse', we are going to get all the lessons in 'lessons' document.

Then in the UI, we can use 'async' pipe to show the lessons:

<md-list>
  <md-list-item *ngFor="let lesson of lessons$ | async">
    <a *ngIf="lesson.hasVideoUrl" [href]="lesson.videoUrl">{{lesson.description}}</a>
    <span *ngIf="!lesson.hasVideoUrl">{{lesson.url}}</span>
  </md-list-item>
</md-list>

  ngOnInit() {
    if(this.route.snapshot.params['url']){
      const url = this.route.snapshot.params['url'];
      this.lessons$ = this.courseService.findAllCourseLessons(url);
    }
  }

Github