title: Add items to ngx-chips input model and only keep display and value fields
body: <p>I'm using <a href="https://www.npmjs.com/package/ngx-chips" rel="nofollow noreferrer">ngx-chips</a> for a tag input control. I'm using my own custom objects as items to use in the control's dropdown; these have this structure:</p>
  
  <pre><code>_id: number;
  name: string;
  summary: string;
  creationDate?: string;
  lastUpdate?: string;
  </code></pre>
  
  <p>So as per ngx-chips documentation, I provide <code>[identifyBy]="'_id'"</code> and <code>[displayBy]="'name'"</code> as inputs to the control; now when I choose another item to add on the control and update the model, it gets added with all of its fields as listed above;</p>
  
  <p>This is my input:</p>
  
  <pre><code>&lt;tag-input [(ngModel)]="goalModel" [theme]="'wsm-theme'" [identifyBy]="'_id'" [displayBy]="'name'" [onlyFromAutocomplete]="true" [placeholder]="'Add filters'" class="wsm-input px-1 py-0"&gt;
       &lt;tag-input-dropdown [showDropdownIfEmpty]="true"
                           [autocompleteItems]="goalFilters"
                           [limitItemsTo]="4"
                           [identifyBy]="'_id'" [displayBy]="'name'"&gt;
       &lt;/tag-input-dropdown&gt;
  &lt;/tag-input&gt;
  </code></pre>
  
  <p>Is there some way to upon adding the item to only keep the <code>_id</code> and <code>name</code> fields on the model? I'm thinking it should be something done in the <code>(onAdd)</code> output event of the control; but I can't think of a way to actually make this happen, if it can be done at all.</p>