fix(select): throw for selectAs and trackBy

trackBy and selectAs have never worked together, and are fundamentally
incompatible since model changes cannot deterministically be
reflected back to the view. This change throws an error to help
developers better understand this scenario.

Author: jeffbcross

docs/content/error/ngOptions/trkslct.ngdoc

@@ -0,0 +1,28 @@
+@ngdoc error
+@name ngOptions:trkslct
+@fullName Comprehension expression cannot contain both selectAs and trackBy expressions.
+@description
+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'};`
+ * trackBy 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 `trackBy` to the values in the array, e.g.
+      In the example: [1,2]
+   2. apply `trackBy` to the already selected value in `ngModel`:
+      In the example: this is not possible, as `trackBy` refers to `item.id`, but the selected
+      value from `ngModel` is `{name: aSubItem}`.
+
+Here's an example of correct syntax:
+
+```
+//track by with array
+<select ng-model="selected" ng-options="item.label for item in values track by item.id">
+```
+
+For more information on valid expression syntax, see 'ngOptions' in {@link ng.directive:select select} directive docs.

src/ng/directive/select.js

@@ -80,8 +80,7 @@ var ngOptionsMinErr = minErr('ngOptions');
 
  * <div class="alert alert-info">
  * **Note:** Using `selectAs` together with `trackexpr` is not possible (and will throw).
- * TODO: Add some nice reasoning here, add a minErr and a nice error page.
- * reasoning:
+ * Reasoning:
  * - Example: <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: 'bSubItemß'}}],
  *   $scope.selected = {name: 'aSubItem'};
@@ -367,6 +366,13 @@ var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
             //re-usable object to represent option's locals
             locals = {};
 
+        if (trackFn && selectAsFn) {
+          throw ngOptionsMinErr('trkslct',
+            "Comprehension expression cannot contain both selectAs ('{0}') " +
+            "and trackBy ('{1}') expressions.",
+            selectAs, track);
+        }
+
         if (nullOption) {
           // compile the element since there might be bindings in it
           $compile(nullOption)(scope);

test/ng/directive/selectSpec.js

@@ -663,7 +663,7 @@ describe('select', function() {
     });
 
 
-    describe('trackBy', function() {
+    describe('trackBy expression', function() {
       beforeEach(function() {
         scope.arr = [{id: 10, label: 'ten'}, {id:20, label: 'twenty'}];
         scope.obj = {'10': {score: 10, label: 'ten'}, '20': {score: 20, label: 'twenty'}};
@@ -761,173 +761,22 @@ describe('select', function() {
     });
 
 
-    describe('selectAs+trackBy', function() {
+    describe('selectAs+trackBy expression', function() {
       beforeEach(function() {
         scope.arr = [{id: 10, label: 'ten'}, {id:'20', label: 'twenty'}];
         scope.obj = {'10': {score: 10, label: 'ten'}, '20': {score: 20, label: 'twenty'}};
       });
 
 
-      it('should bind selectAs expression result to scope (array&single)', function() {
-        createSelect({
-          'ng-model': 'selected',
-          'ng-options': 'item.id as item.name for item in values track by item.id'
-        });
-
-        scope.$apply(function() {
-          scope.values = [{id: 10, name: 'A'}, {id: 20, name: 'B'}];
-          scope.selected = 10;
-        });
-        expect(element.val()).toEqual('0');
-
-        scope.$apply(function() {
-          scope.selected = 20;
-        });
-        expect(element.val()).toEqual('1');
-
-        element.val('0');
-        browserTrigger(element, 'change');
-        expect(scope.selected).toBe(10);
-      });
-
-
-      it('should bind selectAs expression result to scope (array&multiple)',function() {
-        createSelect({
-          'ng-model': 'selected',
-          'multiple': true,
-          'ng-options': 'item.id as item.name for item in values track by item.id'
-        });
-
-        scope.$apply(function() {
-          scope.values = [{id: 10, name: 'A'}, {id: 20, name: 'B'}];
-          scope.selected = [10];
-        });
-        expect(element.val()).toEqual(['0']);
-
-        scope.$apply(function() {
-          scope.selected = [20];
-        });
-        expect(element.val()).toEqual(['1']);
-
-        element.children(0).attr('selected', 'selected');
-        element.children(1).attr('selected', 'selected');
-        browserTrigger(element, 'change');
-        expect(scope.selected).toEqual([10, 20]);
-      });
-
-
-      it('should bind selectAs expression result to scope (object&single)', function() {
-        createSelect({
-          'ng-model': 'selected',
-          'ng-options': 'value.score as value.label for (key, value) in obj track by value.score'
-        });
-
-        scope.$apply(function() {
-          scope.selected = 10;
-        });
-        expect(element.val()).toEqual('10');
-
-        scope.$apply(function() {
-          scope.selected = 20;
-        });
-        expect(element.val()).toEqual('20');
-
-        element.val('10');
-        browserTrigger(element, 'change');
-        expect(scope.selected).toBe(10);
-      });
-
-
-      it('should bind selectAs expression result to scope (object&multiple)', function() {
-        createSelect({
-          'ng-model': 'selected',
-          'multiple': true,
-          'ng-options': 'value.score as value.label for (key, value) in obj track by value.score'
-        });
-
-        scope.$apply(function() {
-          scope.selected = [10];
-        });
-        expect(element.val()).toEqual(['10']);
-
-        scope.$apply(function() {
-          scope.selected = [20];
-        });
-        expect(element.val()).toEqual(['20']);
-
-        element.find('option')[0].selected = 'selected';
-        browserTrigger(element, 'change');
-        expect(scope.selected).toEqual([10, 20]);
-      });
-
-
-      it('should correctly assign model if track & select expressions differ (array&single)', function() {
-        createSelect({
-          'ng-model': 'selected',
-          'ng-options': 'item.label as item.label for item in arr track by item.id'
-        });
-
-        scope.$apply(function() {
-          scope.selected = 'ten';
-        });
-        expect(element.val()).toBe('0');
-
-        element.val('1');
-        browserTrigger(element, 'change');
-        expect(scope.selected).toBe('twenty');
-      });
-
-
-      it('should correctly assign model if track & select expressions differ (array&multiple)', function() {
-        createSelect({
-          'ng-model': 'selected',
-          'multiple': true,
-          'ng-options': 'item.label as item.label for item in arr track by item.id'
-        });
-
-        scope.$apply(function() {
-          scope.selected = ['ten'];
-        });
-        expect(element.val()).toEqual(['0']);
-
-        element.find('option')[1].selected = 'selected';
-        browserTrigger(element, 'change');
-        expect(scope.selected).toEqual(['ten', 'twenty']);
-      });
-
+      it('should throw a helpful minerr', function() {
+        expect(function() {
 
-      it('should correctly assign model if track & select expressions differ (object&single)', function() {
-        createSelect({
-          'ng-model': 'selected',
-          'ng-options': 'val.label as val.label for (key, val) in obj track by val.score'
-        });
-
-        scope.$apply(function() {
-          scope.selected = 'ten';
-        });
-        expect(element.val()).toBe('10');
-
-        element.val('20');
-        browserTrigger(element, 'change');
-        expect(scope.selected).toBe('twenty');
-      });
-
-
-      it('should correctly assign model if track & select expressions differ (object&multiple)', function() {
-        createSelect({
-          'ng-model': 'selected',
-          'multiple': true,
-          'ng-options': 'val.label as val.label for (key, val) in obj track by val.score'
-        });
-
-        scope.$apply(function() {
-          scope.selected = ['ten'];
-        });
-        expect(element.val()).toEqual(['10']);
+            createSelect({
+              'ng-model': 'selected',
+              'ng-options': 'item.id as item.name for item in values track by item.id'
+            });
 
-        element.find('option')[1].selected = 'selected';
-        browserTrigger(element, 'change');
-        expect(scope.selected).toEqual(['ten', 'twenty']);
+        }).toThrowMinErr('ngOptions', 'trkslct', "Comprehension expression cannot contain both selectAs ('item.id') and trackBy ('item.id') expressions.")
       });
     });
 
@@ -1264,7 +1113,7 @@ describe('select', function() {
       it('should bind to scope value and track/identify objects', function() {
         createSelect({
           'ng-model': 'selected',
-          'ng-options': 'item as item.name for item in values track by item.id'
+          'ng-options': 'item.name for item in values track by item.id'
         });
 
         scope.$apply(function() {