How to retrive multiple selected checkbox in edit form which is selected checkboxes at the time add form. in angular 7

I am selected checkbox which is i want in Units and i post it to database by using angular 6. But at the time of get selected Unit checkbox by use [checked]="is_selected" not working means its not shows me which is i selected checkbox only all units are shown.

I already tried to getByProduct and tries also to check which is selected Unit in my product and is selected value = Unit_id in Unit class then its get true. its also changed to true in console. But i have problem its not changed in Unit class is_selected is true or false. and I cant initialise the Unit class to constructor as new Unit() to work fine

                                  Product.component.ts
  Productitems: Items[];
  units: Array<Unit> = [];
  Unititems : Unit[];
  complexForm: FormGroup;
  formData: FormData;
  model : Items;
  unit_id: number[] = [];

  constructor() 
  {
    this.model = new Items();
   }

getProductByID(Id: string): Items {
  let params: any = {};
  params.product_id = Id;
  this.unit_id = [];
  this.Item_Service.getAllUnits().subscribe(response => {
  this.units = response;
   this.Item_Service.getProducts_ById(params).subscribe(
    responce => {
     this.model = responce;
     this.units.forEach(x => {
      this.model.unit_id.forEach(y => {
        if (x.unit_id == y) {
          x.IsSelected = true;
          this.unit_id.push(x.unit_id);
     }
    })
   })
  },
 err => { console.log(err); });
 },
 err => { console.log(err); });
  return this.model;
}

export class Unit {
    public IsSelected: boolean;
    constructor() {
        this.IsSelected = false;
    }
}

<div class="col-lg-3 col-md-4 col-sm-4 col-xs-12"  *ngFor="let item of Unititems">
 <div class="form-group">
  <div class="checkbox checkbox-primary">
   <input id="checkbox"class="styled" type="checkbox"
     (change)="onChangeunit_id(item.unit_id, $event.target.checked)"
      [checked]="IsSelected">
    <label for="checkbox">
         
    </label>
   </div>
  </div>
</div>

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