AngularFire2: Realtime Database: how to get key and value

问题

I use AngularFire2 to get data from Firebase Database (realtime).

What I have done:

• Firebase Database

{ “class” : { “student” : { “Tom” : “male”, “Mary” : “female”, “Peter” : “male”, “Laura” : “female” }, "numberOfStudent” : 10 } }

• app.component.ts

  import { AngularFireDatabase } from 'angularfire2/database';
  import { Observable } from 'rxjs/Observable';
  
  ...
  export class AppComponent {
  
     class: Observable<any>;
     students: Observable<any[]>;
  
  constructor(private db: AngularFireDatabase) {
     this.class = db.object(‘class’).valueChanges();
     this.students = db.list(‘class/student’).snapshotChanges();
   }
  
  } 
  

• app.component.html:

<h2>Class size: {{ (class | async)?.numberOfStudent }}</h2>
<ul>
  <li *ngFor="let i of students | async">
    {{i.key}} : {{i.value}}
  </li>
</ul>

What happened:

Class size: 10

Tom :

Mary :

Peter :

Laura :

It doesn't return the value of list.

Any suggestion is appreciated.

回答1:

UPD

with new Angular 6 and RxJS 6 you'll do this:

import { map } from 'rxjs/operators';
// ...
return this.fb.list(`/path/to/list`)
  .snapshotChanges()
  .pipe(map(items => { .            // <== new way of chaining
    return items.map(a => {
      const data = a.payload.val();
      const key = a.payload.key;
      return {key, data};           // or {key, ...data} in case data is Obj
    });
  }));

回答2:

AngularFire2 library has gone through some breaking changes since @rickdroio's answer. The following is an updated version of the solution:

afs.collection<Shirt>('class/student').snapshotChanges().map(actions => {
  return actions.map(a => {
    const data = a.payload.val();
    const id = a.payload.id;
    return { id, ...data };
  });
});

回答3:

I managed to get the key and value of list. Just follow some tips below:

• Make sure using snapshotChanges()

<li *ngFor="let i of seats | async">
    {{i.key}} : {{i.payload.val()}}
</li>

It worked for me, but I am still opening to receive more best practices

回答4:

According to guideline available on https://github.com/angular/angularfire2/blob/master/docs/firestore/collections.md you could do something like that:

afs.collection<Shirt>('class/student').snapshotChanges().map(actions => {
      return actions.map(a => {
        const data = a.payload.doc.data();
        const id = a.payload.doc.id;
        return { id, ...data };
      });
    });

It will return a similar array like previous Firebase db.

回答5:

your problem is your JSON object students is not an array, and you are trying to loop through it.

    "student" : { “Tom” : “male”, “Mary” : “female”, “Peter” : “male”, “Laura” :
“female” }, "numberOfStudent” : 10 }

you need to make your students a list of objects in order to loop through them, like so:

   "student" :
[ { "name" : "Tom", "sex" : male}, {"name" : "Mary, "sex" : "female" }, ... ]

the loop through let i of student | async and access the name and sex i?.name, i?.sex

回答6:

With Angular 6
Breakdown:

1. I created a Map to store the key/values for future queries.

2. Fetched the values and added them to the previously created Map

3. Created two helper methods to get the key or the value separately.

todosKeyValues: Map<string, Todo> = new Map();

constructor(private databaseFB: AngularFireDatabase) {
    this.fetchKeyValues();
 }

private fetchKeyValues() {
    this.databaseFB.list('/').snapshotChanges().subscribe(actions => {
      actions.forEach(action => {
        const value = action.payload.val();
        const id = action.payload.key;
        this.todosKeyValues.set(id, value);
      });
    });
  }


 private getKey(id: number): string {
 const foundEntry = Array.from(this.todosKeyValues.entries())
    .filter(entry =>entry[1].id === id).pop();
   return foundEntry ? foundEntry[0] : undefined;
  }

  private getTodo(id: number): Todo {
    const foundEntry = Array.from(this.todosKeyValues.entries())
    .filter(entry => entry[1].id === id).pop();
      //index 0 is the key, index 1 is the value
    return foundEntry ? foundEntry[1] : undefined;
  }
  ...

来源：https://stackoverflow.com/questions/46587495/angularfire2-realtime-database-how-to-get-key-and-value