How to get involved in angularfire2 database

Question

I searched and I still could not get the right answer. I will be happy that you will point me in the right direction.

I have a database with users and projects

pojects: 
   08177a0acb3f4d96f8cb5fa19002d2ed:
       pid: 08177a0acb3f4d96f8cb5fa19002d2ed,
       pName: "Prject 1"
       uId:  "254",
       createdAt: 9377476



users: 
   254:
       userName: "Eric"
       uId:  "254"
       avatar: "image.jpg"

Now I want to display my projects, I get a list in my service

this.projectList = this.af.database.list('projects',  {
      query: {
        orderByChild: 'createdAt'
      }
    });



getProjects() {
     return this.projectList.map(snapshot => {
         return snapshot;
     });

and I subscribe to component.ts

this.projectService.getProjects()
    .subscribe(projects => this.projects = projects);

What I want to do is get userName from users and display it with projects.

How can I use the uId field in projects to retrieve user information in a user list into a single list so that I can display ie {{project.userName}} {{project.avatar}}?

Answer 1

forkJoin, :

import { Observable } from 'rxjs/Observable';
import 'rxjs/add/observable/forkJoin';
import 'rxjs/add/operator/first';
import 'rxjs/add/operator/switchMap';

// Compose an observable based on the projectList:

this.projectWithUserList = this.projectList

  // Each time the projectList emits, switch to unsubscribe/ignore
  // any pending user queries:

  .switchMap(projects => {

    // Map the projects to the array of observables that are to be
    // joined.

    let userObservables = projects.map(project => this.af.database
      .object(`users/${project.uId}`)
      .first()
    );

    // Join the user observables, and match up the users with the
    // projects, etc.

    return userObservables.length === 0 ?
      Observable.of(projects) :
      Observable.forkJoin(...userObservables, (...users) => {
        projects.forEach((project, index) => {
          project.userName = users[index].userName;
          project.avatar = users[index].avatar;
        });
        return projects;          
      })
  });

, , , .

, , , combineLatest forkJoin :

import { Observable } from 'rxjs/Observable';
import 'rxjs/add/observable/combineLatest';
import 'rxjs/add/operator/switchMap';

// Compose an observable based on the projectList:

this.projectWithUserList = this.projectList

  // Each time the projectList emits, switch to unsubscribe/ignore
  // any pending user queries:

  .switchMap(projects => {

    // Map the projects to the array of observables that are to be
    // combined.

    let userObservables = projects.map(project => this.af.database
      .object(`users/${project.uId}`)
    );

    // Combine the user observables, and match up the users with the
    // projects, etc.

    return userObservables.length === 0 ?
      Observable.of(projects) :
      Observable.combineLatest(...userObservables, (...users) => {
        projects.forEach((project, index) => {
          project.userName = users[index].userName;
          project.avatar = users[index].avatar;
        });
        return projects;          
      });
  });