Nested Angular2 ngFor Directives for Firebase Lists [duplicate]

Question

I want to bind the results of a Firebase query to my template using ngFor in Angular 2. This is easily achieved below.

component:

export class FeaturedThreadsComponent { 
    threads: FirebaseListObservable<any[]>;
    qualitySubject: Subject<any>;

    constructor(af: AngularFire) {
        this.qualitySubject = new Subject();
        this.threads = af.database.list('/threads', {
            query: {
                orderByChild: 'quality',
                endAt: 5
            }
        });
    }
}

template:

<div *ngFor="let thread of threads | async">
    {{thread.title}}
</div>

But if I want to use another ngFor directive nested within the template to list a child object's keys...

<div *ngFor="let thread of threads | async">
    {{thread.title}}
    <div *ngFor="let participant of thread.participants">
        {{participant.$key}}}
    </div>
</div>

I get the console error, Cannot find a differ supporting object '[object Object]' of type 'object'. NgFor only supports binding to Iterables such as Arrays.

In my database structure, participants is a child of thread, which is a child of threads where thread is a dynamic key. So I can't use a direct path to participants.

This pattern of nesting ngFor directives worked fine in a service that simply iterated over a local file. Why it isn't working here seems a bit fuzzy to me.

Answer 1

For those of you who follow this thread to the one it is flagged as a duplicate of, resist creating a pipe like the accepted answer suggests. The best answer avoids the performance issues of the accepted answer and is much simpler.