Implementing custom client validation based off of another textbox input - asp.net-mvc-5

Question

I've been dabbling a little into MVC 5's custom client validation (MVC's GetClientValidationRules and jQuery's validator). I've been able to successfully implement a validator to check whether the user selects a date that is in the past. But what about if I need to check if a user's textbox input is greater than another textbox's input of theirs?
I have it working fine without unobtrusive validation/server-side, but not with unobtrusive validation.
Here's an example.
Model
public Nullable<int> ItemsPurchased
public Nullable<int> ItemsReturned
A custom DataAnnotation has been made for ItemsReturned to check whether its value <= ItemsPurchased. Implementing GetClientValidationRules to actually get ItemsPurchased's current value is where I'm having trouble in the code.

Answer 1

I had a variable in my custom data annotation. I'm not sure why this didn't come to me naturally...but here's my solution.
DataAnnotation (Applied to ItemsReturned)
public string purchasedQuantityField { get; set; }
public ReturnedAttribute(string quantity) {
purchasedQuantityField = quantity;
}
...
(in GetClientValidationRules)
ModelClientvalidationRule purchaseRule = new ModelClientValidationRule();
purchaseRule.ValidationType = "purchaserestriction";
purchaseRule.ErrorMessage = ErrorMessageString;
purchaseRule.ValidationParameters["otherfield"] = purchasedQuantityField;
Usage in the model:
[Returned("ItemsPurchased", ErrorMessage = "Returned Items cannot be greater than the number of Items Purchased.")]
Then I made my own JS file with the custom client validation rules, CustomClientValidation.js:
jQuery.validator.addMethod("purchaserestriction", function (value, element, params) {
var purchasedFieldVal = $('input[name="' + params.otherfield + '"]').val();
if (purchasedFieldVal && value) {
var returnedVal = parseInt(value);
var purchasedVal = parseInt(purchasedFieldVal);
if (returnedVal <= purchasedVal) return true;
else return false;
}
);
jQuery.validator.unobtrusive.adapters.add("purchaserestriction", ["otherfield"], function (options) {
options.rules["purchaserestriction"] = options.params;
if (options.message) options.messages["purchaserestriction"] = options.message;
});

Answer 2

It's possible with jQuery Validation. You can create custom rules which will check whatever you want. Take a look at this:
$.validator.addMethod('name_of_the_rule', function (value, element, param) {
//put there your validation
return isValid; // return bool here if valid or not.
}, 'Put here your error message!');
$('#form').validate({
rules: {
field1: {
yourRuleName: true
}
}
});