title: Filter on a mat-select
body: <p>I need to filter data on a select because there is too much options.
  I did it like this :</p>
  
  <pre><code>        &lt;mat-form-field&gt;
            &lt;mat-select placeholder="Unit.." (change)="onTabChange(element, $event.value.id, 'assignedUnit')"&gt;
              &lt;input matInput type="text" (keyup)="filterListCareUnit($event.target.value)"&gt;
              &lt;mat-option *ngFor="let careUnit of listCareUnits" [value]="careUnit"&gt;
                {{ careUnit.label }}
              &lt;/mat-option&gt;
            &lt;/mat-select&gt;
          &lt;/mat-form-field&gt;
  </code></pre>
  
  <p>on key up, i'm calling <code>filterListCareUnit($event.target.value)</code></p>
  
  <p>but in this function i don't know to use the filter rxjs</p>
  
  <p>I have my <code>listCareUnits</code> and I want to remove all object which don't contains the <code>$event.target.value</code></p>
  
  <p>for now I did this but clearly don't work, my list always contains the same items :</p>
  
  <pre><code>  filterListCareUnit(val) {
      console.log(val);
      this.listCareUnits.filter((unit) =&gt; unit.label.indexOf(val));
    }
  </code></pre>
  
  <p>Newbie at angular/rxjs ..
  Thanks for the help</p>