feat(select): match options by expression other than object identity

Extend ng-options with a new clause, "track by [trackByExpression]", which can be used when
working with objects.  The `trackByExpression` should uniquely identify select options objects.
This solves the problem of previously having to match ng-options objects by identity.
You can now write: `ng-options="obj as obj.name for obj in objects track by obj.id"`
The "track by" expression will be used when checking for equality of objects.

Examples:
<select
    ng-model="user.favMovieStub"
    ng-options="movie as movie.name for movie in movies track by movie.id">
</select>

scope: {
  user: { name: 'Test user', favMovieStub: { id: 1, name: 'Starwars' } }
  movies: [{ id: 1, name: 'Starwars', rating: 5, ... }, { id: 13, ... }]
}

The select input will match user favMovieStub to the first movie in the movies array, and show
"Star Wars" as the selected item.

src/ng/directive/select.js

@@ -39,7 +39,7 @@
  *     * `label` **`for`** `value` **`in`** `array`
  *     * `select` **`as`** `label` **`for`** `value` **`in`** `array`
  *     * `label`  **`group by`** `group` **`for`** `value` **`in`** `array`
- *     * `select` **`as`** `label` **`group by`** `group` **`for`** `value` **`in`** `array`
+ *     * `select` **`as`** `label` **`group by`** `group` **`for`** `value` **`in`** `array` **`track by`** `trackexpr`
  *   * for object data sources:
  *     * `label` **`for (`**`key` **`,`** `value`**`) in`** `object`
  *     * `select` **`as`** `label` **`for (`**`key` **`,`** `value`**`) in`** `object`
@@ -59,6 +59,9 @@
  *      element. If not specified, `select` expression will default to `value`.
  *   * `group`: The result of this expression will be used to group options using the `<optgroup>`
  *      DOM element.
+ *   * `trackexpr`: Used when working with an array of objects. The result of this expression will be
+ *      used to identify the objects in the array. The `trackexpr` will most likely refer to the
+ *     `value` variable (e.g. `value.propertyName`).
  *
  * @example
     <doc:example>
@@ -123,8 +126,8 @@
 
 var ngOptionsDirective = valueFn({ terminal: true });
 var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
-                         //0000111110000000000022220000000000000000000000333300000000000000444444444444444440000000005555555555555555500000006666666666666666600000000000000077770
-  var NG_OPTIONS_REGEXP = /^\s*(.*?)(?:\s+as\s+(.*?))?(?:\s+group\s+by\s+(.*))?\s+for\s+(?:([\$\w][\$\w\d]*)|(?:\(\s*([\$\w][\$\w\d]*)\s*,\s*([\$\w][\$\w\d]*)\s*\)))\s+in\s+(.*)$/,
+                         //0000111110000000000022220000000000000000000000333300000000000000444444444444444440000000005555555555555555500000006666666666666666600000000000000007777000000000000000000088888
+  var NG_OPTIONS_REGEXP = /^\s*(.*?)(?:\s+as\s+(.*?))?(?:\s+group\s+by\s+(.*))?\s+for\s+(?:([\$\w][\$\w\d]*)|(?:\(\s*([\$\w][\$\w\d]*)\s*,\s*([\$\w][\$\w\d]*)\s*\)))\s+in\s+(.*?)(?:\s+track\s+by\s+(.*?))?$/,
       nullModelCtrl = {$setViewValue: noop};
 
   return {
@@ -298,7 +301,7 @@ var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
 
         if (! (match = optionsExp.match(NG_OPTIONS_REGEXP))) {
           throw Error(
-            "Expected ngOptions in form of '_select_ (as _label_)? for (_key_,)?_value_ in _collection_'" +
+            "Expected ngOptions in form of '_select_ (as _label_)? for (_key_,)?_value_ in _collection_ (track by _expr_)?'" +
             " but got '" + optionsExp + "'.");
         }
 
@@ -308,6 +311,8 @@ var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
             groupByFn = $parse(match[3] || ''),
             valueFn = $parse(match[2] ? match[1] : valueName),
             valuesFn = $parse(match[7]),
+            track = match[8],
+            trackFn = track ? $parse(match[8]) : null,
             // This is an array of array of existing option groups in DOM. We try to reuse these if possible
             // optionGroupsCache[0] is the options with no option group
             // optionGroupsCache[?][0] is the parent: either the SELECT or OPTGROUP element
@@ -348,7 +353,14 @@ var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
                   if ((optionElement = optionGroup[index].element)[0].selected) {
                     key = optionElement.val();
                     if (keyName) locals[keyName] = key;
-                    locals[valueName] = collection[key];
+                    if (trackFn) {
+                      for (var trackIndex = 0; trackIndex < collection.length; trackIndex++) {
+                        locals[valueName] = collection[trackIndex];
+                        if (trackFn(scope, locals) == key) break;
+                      } 
+                    } else {
+                      locals[valueName] = collection[key];
+                    }
                     value.push(valueFn(scope, locals));
                   }
                 }
@@ -360,9 +372,19 @@ var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
               } else if (key == ''){
                 value = null;
               } else {
-                locals[valueName] = collection[key];
-                if (keyName) locals[keyName] = key;
-                value = valueFn(scope, locals);
+                if (trackFn) {
+                  for (var trackIndex = 0; trackIndex < collection.length; trackIndex++) {
+                    locals[valueName] = collection[trackIndex];
+                    if (trackFn(scope, locals) == key) {
+                      value = valueFn(scope, locals);
+                      break;
+                    }
+                  }
+                } else {
+                  locals[valueName] = collection[key];
+                  if (keyName) locals[keyName] = key;
+                  value = valueFn(scope, locals);
+                }
               }
             }
             ctrl.$setViewValue(value);
@@ -394,7 +416,15 @@ var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
               label;
 
           if (multiple) {
-            selectedSet = new HashMap(modelValue);
+            if (trackFn && isArray(modelValue)) {
+              selectedSet = new HashMap([]);
+              for (var trackIndex = 0; trackIndex < modelValue.length; trackIndex++) {
+                locals[valueName] = modelValue[trackIndex];
+                selectedSet.put(trackFn(scope, locals), modelValue[trackIndex]);
+              }
+            } else {
+              selectedSet = new HashMap(modelValue);
+            }
           }
 
           // We now build up the list of options we need (we merge later)
@@ -406,15 +436,21 @@ var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
               optionGroupNames.push(optionGroupName);
             }
             if (multiple) {
-              selected = selectedSet.remove(valueFn(scope, locals)) != undefined;
+              selected = selectedSet.remove(trackFn ? trackFn(scope, locals) : valueFn(scope, locals)) != undefined;
             } else {
-              selected = modelValue === valueFn(scope, locals);
+              if (trackFn) {
+                var modelCast = {};
+                modelCast[valueName] = modelValue;
+                selected = trackFn(scope, modelCast) === trackFn(scope, locals);
+              } else {
+                selected = modelValue === valueFn(scope, locals);
+              }
               selectedSet = selectedSet || selected; // see if at least one item is selected
             }
             label = displayFn(scope, locals); // what will be seen by the user
             label = label === undefined ? '' : label; // doing displayFn(scope, locals) || '' overwrites zero values
             optionGroup.push({
-              id: keyName ? keys[index] : index,   // either the index into array or key from object
+              id: trackFn ? trackFn(scope, locals) : (keyName ? keys[index] : index),   // either the index into array or key from object
               label: label,
               selected: selected                   // determine if we should be selected
             });

test/ng/directive/selectSpec.js

@@ -495,7 +495,7 @@ describe('select', function() {
       expect(function() {
         compile('<select ng-model="selected" ng-options="i dont parse"></select>');
       }).toThrow("Expected ngOptions in form of '_select_ (as _label_)? for (_key_,)?_value_ in" +
-                 " _collection_' but got 'i dont parse'.");
+                 " _collection_ (track by _expr_)?' but got 'i dont parse'.");
     });
 
 
@@ -753,6 +753,37 @@ 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'
+        });
+
+        scope.$apply(function() {
+          scope.values = [{id: 1, name: 'first'},
+                          {id: 2, name: 'second'},
+                          {id: 3, name: 'third'},
+                          {id: 4, name: 'forth'}];
+          scope.selected = {id: 2};
+        });
+
+        expect(element.val()).toEqual('2');
+
+        var first = jqLite(element.find('option')[0]);
+        expect(first.text()).toEqual('first');
+        expect(first.attr('value')).toEqual('1');
+        var forth = jqLite(element.find('option')[3]);
+        expect(forth.text()).toEqual('forth');
+        expect(forth.attr('value')).toEqual('4');
+
+        scope.$apply(function() {
+          scope.selected = scope.values[3];
+        });
+
+        expect(element.val()).toEqual('4');
+      });
+
+
       it('should bind to scope value through experession', function() {
         createSelect({
           'ng-model': 'selected',