Angular Js：您能否在ng-options中结合使用“ track by”来设置选择的默认值

Question

The best method to set the values for your options in your select list, seems to be to use 'track-by' (using a.id as a.name seems to be out-dated) in your ng-options directive. However, when I use 'track by', I am no longer able to set the default value using ng-model.

Take for example, this plunkr: http://plnkr.co/edit/OZCal9ZkCQeqnQJY0WP9?p=preview

  <select ng-model="class.team.leader" ng-options="student.name for student in curTeam.students track by student._id">

Currently, the plunkr correctly sets the default value. However, if you change it to use 'track by', like in the code above, it does not work.

Answer 1

track by doesn't work like that. It's used to compare a property on both objects rather than checking for object equality. ie obj1.trackBy === obj2.trackBy rather than obj1 === obj2 .

For example, change your default model to:

$scope.class = {
    team: {
       leader: { _id: 2, name: 'Dave', status: 0 }
    }  
};

Without track by this wouldn't work as it's a different object to what's in the students array. But with track by angular will compare the _id properties to decide equality.