title: How do I set Angular 2 ngModel from a firebase observable on a select dropdown
body: <p>I am trying to set the default value in a select dropdown from a firebase object using angularfire 2. But I don't know how to make it work with setting the default value of the select box. ngModel doesn't allow something like <code>(ngModel)="(default_tax | async)"</code></p>
  
  <p>code:</p>
  
  <pre><code>public default_tax$:Observable&lt;any&gt;;
  public tax$:Observable&lt;Tax&gt;;
  
  ngOnInit(): void {
      this.tax$ = this.db.list(`tax_rates`);
      this.default_tax$ = this.db.object(`settings/default_tax_rate`);
  }
  </code></pre>
  
  <p>template:</p>
  
  <pre><code>&lt;select name="tax_rate" (ngModel)="default_tax.$key" (ngModelChange)="onSelect($event)"&gt;
      &lt;option  *ngFor="let tax of (taxes$ | async)" [ngValue]="tax.$key"&gt;
      {{tax.name}} - {{tax.rate}}
      &lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>Firebase object:</p>
  
  <pre><code>account:
      setttings:
           default_tax_rate: "-somekey"
      tax_rates:
           "-somekey":
                "name":"5.5"
           "-someotherkey"
                 "anothername":"4.5"
  </code></pre>