如何在angularfire2数据库中进行联接 [英] How to get do a join in angularfire2 database

问题描述

我进行了搜索，但仍然找不到正确的答案.我会很高兴您为我指出正确的方向.

I have searched through and I still couldn't get the right answer. I will be happy you point me to the right direction.

我有一个包含用户和项目的数据库

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"

现在我想显示我的项目，我在服务中进行列表检索

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

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



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

我订阅了component.ts

and I subscribe in the component.ts

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

我要做的是从用户那里获取用户名，并与项目一起显示.

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

如何使用项目中的uId字段将用户列表中的用户信息检索到一个列表中，以便显示{{project.userName}} {{project.avatar}}?

How can I use the uId field in projects to retrieve the user info in the users list into one list so I can display i.e {{project.userName}} {{project.avatar}}?

推荐答案

您可以使用forkJoin获取项目的用户，并可以使用其结果选择器将所需的属性复制到发出的项目对象中:

You can use forkJoin to obtain the users for the projects and can use its result selector to copy the required properties into the emitted project objects:

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;          
      })
  });

只要数据库中的项目发生更改，上述实现都会发出一个项目数组，但是如果用户信息发生更改，则不会发出一个数组.

The above implementation will emit an array of projects whenever the projects in the database change, but it will not emit an array if the user information changes.

如果要更改项目或用户信息时发出数组，则可以对用户使用combineLatest而不是forkJoin:

If you want to emit an array if either the projects or the user information changes, you can use combineLatest instead of forkJoin for the users:

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;          
      });
  });

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