fix(ngModel): add some info about

Author: Narretz

src/ng/directive/input.js

@@ -1956,6 +1956,9 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
    *
    * @description
    * Runs each of the registered validators (first synchronous validators and then asynchronous validators).
+   * If the validity changes to invalid, the model will be set to undefined, unless ngModelOptions.allowInvalid
+   * is `true`. If the validity changes to valid, it will set the model to the last available valid
+   * modelValue.
    */
   this.$validate = function() {
     // ignore $validate before model is initialized
@@ -1964,12 +1967,16 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
     }
 
     var viewValue = ctrl.$$lastCommittedViewValue;
-    var modelValue = ctrl.$rawModelValue;
+    var modelValue = ctrl.$$rawModelValue;
+
     var prevValid = ctrl.$valid;
     var prevModelValue = ctrl.$modelValue;
+
     var allowInvalid = ctrl.$options && ctrl.$options.allowInvalid;
 
     ctrl.$$runValidators(undefined, modelValue, viewValue, function(allValid) {
+      // If there was no change in validity, don't update the model
+      // This prevents changing an invalid modelValue to undefined
       if (!allowInvalid && prevValid !== allValid) {
         // Note: Don't check ctrl.$valid here, as we could have
         // external validators (e.g. calculated on the server),
@@ -2130,7 +2137,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
     }
     var prevModelValue = ctrl.$modelValue;
     var allowInvalid = ctrl.$options && ctrl.$options.allowInvalid;
-    ctrl.$rawModelValue = modelValue;
+    ctrl.$$rawModelValue = modelValue;
     if (allowInvalid) {
       ctrl.$modelValue = modelValue;
       writeToModelIfNeeded();
@@ -2255,7 +2262,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
     // if scope model value and ngModel value are out of sync
     // TODO(perf): why not move this to the action fn?
     if (modelValue !== ctrl.$modelValue) {
-      ctrl.$modelValue = ctrl.$rawModelValue = modelValue;
+      ctrl.$modelValue = ctrl.$$rawModelValue = modelValue;
 
       var formatters = ctrl.$formatters,
           idx = formatters.length;