基于选择的角度动态验证

我正在开发一个带有ionic框架和angularJS的移动应用程序。现在我在进行动态验证时遇到了问题。

这是我的表单，当选择框更改时，我试图从控制器设置验证值，但它不起作用。

<form id="my-form" ng-submit="submit(form,request)" name="form" novalidate>
  <div ng-class="{'has-error':form.biller-name.$invalid}">
    <span>Biller</span>
    <select id="biller" name="biller-name" ng-model="request.biller" ng-change="getValidationAmount(request.biller)" required>
        <option value="">Choose biller name</option>
      <option value="BA">Biller A</option>
      <option value="BB">Biller B</option>
        <option value="BC">Biller C</option>
    </select>
    <div ng-show="form.biller-name.$invalid && form.biller-name.$dirty">
        <p ng-if="form.biller-name.$error.required">Biller name must be chosen</p>
    </div>
  </div>
  <div ng-class="{'has-error':form.amount.$invalid}">
    <span>Amount</span>
    <input name="amount" ng-model="request.amount" type="number" ui-number-mask="2" max="{{billerVal.maxAmount}}" min="{{billerVal.minAmount}}" required/>
    <div ng-show="form.amount.$invalid && form.amount.$dirty">
            <p ng-if="form.amount.$invalid">validation works</p>
    </div>
  </div>
  <div>
    <input value="Submit" id="login_btnSubmit" data-role="button"
    data-shadow="false" type="submit"/>
  </div>
</form>

我的JS，有两个功能。首先从数据库获取数据，更新功能将是页面上的更新验证。

$scope.getValidationAmount = function (data){
  //get data from db
  beneficiaryService.getAmountByBenefeciaryType(JSON.parse(data)).then(function(amount){
    var amountSplitLast2Character = amount.substr(amount.length - 2);
    var firstCharacter = amount.substr(0, (amount.length - amountSplitLast2Character.length));
    $scope.bill.amount = firstCharacter+'.'+amountSplitLast2Character;
    //will do update function
    $scope.update(JSON.parse(data).nickname);
  },function(data,opt,error){});
};
$scope.update = function(nickname) {
  var billerVal = {};
  //this code below just for get object biller by nickname
  for(var i in billerList) {
    if (billerList[i].nickname == nickname) {
      for(var j in billerEnumList) {
        if (billerEnumList[j].key == billerList[i].name) {
          billerVal = billerEnumList[j];
        }
      }
    }
  }
  billerVal.maxAmount = parseInt(billerVal.maxAmount); //will return a number of amount e.g 100
  billerVal.minAmount = parseInt(billerVal.minAmount); //will return a number of amount e.g 1
  $scope.billerVal = billerVal; //set validation value to html page
}

当我直接在html中对最小值和最大值进行硬编码验证时，验证工作正常。但是，当我尝试像上面的代码一样使用动态验证时，验证将不起作用。