问无法在Angular中使用Formbuilder设置选择元素中的数组值

EN

onUpdate(invoice) {
 console.log(invoice.customer)

 const control = <FormArray>this.form.controls['purchases'];
 control.controls = [];

 this.form.controls['$key'].setValue(invoice.$key);
 this.form.controls['customer'].setValue(invoice.customer);//this is the problem

 for (let i in invoice.purchases) {
   const product = (invoice.purchases[i].product);
   const quantity = (invoice.purchases[i].quantity);
   this.addPurchase(product);
   control.at(+i).get('quantity').setValue(quantity);
 }
}