title: Angular 6 firebase snapshot returns undefined
body: <p>I am uploading object to my database and then trying to retrieve all items. On the second step I get errors. :</p>
  
  <p><strong>My object class :</strong> </p>
  
  <pre><code>export class Data {
    $key: string;
    name: string;
    address: string;
    address2: string;
    pscode: string;
    ccode: string;
    name2: string;
    trucks: Trucks;
    trailers: Trailers;
    email: string;
    phone: string;
    city: string;
    country: string;
  }
  </code></pre>
  
  <p><strong>My service upload object (works fine) :</strong> </p>
  
  <pre><code>busines = {} as Data;
  
  
  createItemAuth() {
     this.afDatabase.list(`users/${this.auth.userId}/company/`).push(this.busines)
  }
  </code></pre>
  
  <p><strong>My service getUpload :</strong> </p>
  
  <pre><code> getItem: Observable&lt;any[]&gt;;
   getUploads() {
      this.getItem = this.afDatabase.list(`users/${this.auth.userId}/company/`).snapshotChanges().pipe(map(items =&gt; {
        return items.map(a =&gt; {
          const data = a.payload.val();
          const $key = a.payload.key;
          const $ref = a.payload.ref;
          return { $key, ...data, $ref };
        });
      }));
      return this.getItem;
    }
  </code></pre>
  
  <p><strong>Calling it in component :</strong> </p>
  
  <pre><code>uploads: Observable&lt;Data[]&gt;;
  ngOnInit() {
     this.uploads = this.back.getUploads();
     console.log(this.back.getUploads())
  }                                                           
  </code></pre>
  
  <p><strong>HTML: (nothing at all at browser)</strong> </p>
  
  <pre><code>&lt;div *ngFor="let info of uploads | async"&gt;
    &lt;p&gt;{{info.name}}&lt;/p&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p><strong>Console.log on ngOnInit() :</strong></p>
  
  <blockquote>
    <p>Observable {_isScalar: false, source: Observable, operator:
    MapOperator} operator: MapOperator {project: ƒ, thisArg: undefined}
    source: Observable {_isScalar: false, _subscribe: ƒ}
    _isScalar: false
    <strong>proto</strong>: Object</p>
  </blockquote>
  
  <p>Versioning : </p>
  
  <pre><code>"rxjs": "^6.1.0",
    "firebase": "^5.4.1",
    "@angular/cli": "6.0.0",
      "typescript": "2.7.2",
  </code></pre>