Added ngFor track by DSO's id directive

src/app/shared/ng-for-track-by-id.directive.ts

@@ -0,0 +1,16 @@
+import { Directive, Host } from '@angular/core';
+import { NgForOf } from '@angular/common';
+
+import { DSpaceObject } from '../core/shared/dspace-object.model';
+
+@Directive({
+  // tslint:disable-next-line:directive-selector
+  selector: '[ngForTrackById]',
+})
+export class NgForTrackByIdDirective<T extends DSpaceObject> {
+
+  constructor(@Host() private ngFor: NgForOf<T>) {
+    this.ngFor.ngForTrackBy = (index: number, dso: T) => (dso) ? dso.id : undefined;
+  }
+
+}