Angular ngModel with dynamic parameter

Question

In this Angular example, the ngModel binding does not work. Can someone let me understand why? and if so, What is the best way to do it?

(note that the binding uses a dynamic key)

ts:

constructor() {  this.days = { day1:'data', day2:'data2', day3:'data3...' }}

html:

<div *ngFor="let i of [1,2,3]">
   <span> {{days['day'+i]}}</span>
   <input type="text" [(ngModel)]="days['day'+i]" />
</div>

Thank you!