Knockout validation: Dynamic constraints

Question

I'm using Durandal, which in turn leverages off of Knockout.

I want to be able to Change validation lengths dynamically

Fiddle

The fiddle seems to be behaving slightly different than my "working" solution, but its still not doing what I'm wanting/expecting it to.

Viewmodel JS:

[Attempt 1]

define(function () {

   var self = this;

   self.userInfo = {       
        IdOrPassportNumber: ko.observable().extend({
            required: true,
            pattern: {
                message: 'A message',
                params: /some regex/
            }
        }),
        IdType: ko.observable()
    },

    self.isIdValid = ko.validatedObservable({ 
         IdOrPassportNumber: self.userInfo.IdOrPassportNumber 
    });

    self.userInfo.IdOrPassportNumber.subscribe(function (value) {
          if (isIdValid.isValid()) {
               console.log('YOLO!');
          }
    });

    self.userInfo.IdType.subscribe(function (value) {
        console.log(value);
        if (value === 'Passport') {
            self.userInfo.IdOrPassportNumber.extend({ maxLength: 15 });
        } else {
            self.userInfo.IdOrPassportNumber.extend({ maxLength: 13 });
        }
    });    

    var viewModel = {
        userInfo: self.userInfo
    };

    viewModel["errors"] = ko.validation.group(viewModel.userInfo);
    viewModel["errors"].showAllMessages();

    return viewModel;
});

What seems to be happening is that when i start typing i get the max & min validation of 13, but if i continue typing the validation changes to 15. I have tried another route of, setting the min & max length in the initial observable extend EG, just after the regex, and then setting the min and max length to use an observable, to no success.

[Attempt 2]

   self.userInfo = {       
       IdOrPassportNumber: ko.observable().extend({               
            maxLength: self.maxLength(), 
            minlength: self.maxLength()
       }),
       IdType: ko.observable()
   },

   self.maxLength = ko.observable();

   self.userInfo.IdType.subscribe(function (value) {

          if (value === 'Passport') {
             self.maxLength(15)
          } else {
              self.maxLength(3)
          }
    });

Answer 1

Here is the solution that worked for me:

I made use of the custom validation feature, more specifically the single use custom validation as this wont be re-used elsewhere.

[Attempt 3]

    self.userInfo = {    
        IdOrPassportNumber: ko.observable().extend({
            required: true,
            pattern: {
                message: 'A message',
                params: /some regex/
            },
            validation: {
               validator: function (val) {
                   if (self.userInfo.IdType() === 'Id') { 
                      return val.length === 13; 
                   } else { 
                      return val.length === 15; 
                   }
                },
               message: function () {
                  if (self.userInfo.IdType() === 'Id') {
                    return 'Required: 13 characters';
                  } else {
                    return 'Required: 15 characters';
                  }
               }
            }
        })
     }