ngModel缺省是by reference,

1) 缺省是 by reference, not value，  ngModel， ngOptions等都一样

 

2) 如果要 track value 用 "track by"，  ngOptions will track the identity of the item not by reference, but by the result of the track by expression. For example, if your collection items have an id property, you would track by item.id

 

 

 

“select as”  will bind the result of the select expression to the model,