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# Error: error:trkslct
Comprehension expression cannot contain both `select as` and `track by` expressions.

## Description

NOTE: This error was introduced in 1.3.0-rc.5, and was removed for 1.3.0-rc.6 in order to not break existing apps.

This error occurs when 'ngOptions' is passed a comprehension expression that contains both a `select as` expression and a `track by` expression. These two expressions are fundamentally incompatible.

- Example of bad expression: `<select ng-options="item.subItem as item.label for item in values track by item.id" ng-model="selected">` `values: [{id: 1, label: 'aLabel', subItem: {name: 'aSubItem'}}, {id: 2, label: 'bLabel', subItem: {name: 'bSubItem'}}]`, `$scope.selected = {name: 'aSubItem'};`
- track by is always applied to `value`, with purpose to preserve the selection, (to `item` in this case)
- To calculate whether an item is selected, `ngOptions` does the following:
  1. apply `track by` to the values in the array: In the example: [1,2]
  2. apply `track by` to the already selected value in `ngModel`: In the example: this is not possible, as `track by` refers to `item.id`, but the selected value from `ngModel` is `{name: aSubItem}`.

Here's an example of how to make this example work by using `track by` without `select as`:

    <select ng-model="selected" ng-options="item.label for item in values track by item.id">

Note: This would store the whole `item` as the model to `scope.selected` instead of `item.subItem`.

For more information on valid expression syntax, see 'ngOptions' in select directive docs.