{{input type="text" value=firstname maxlength=50}}

var CONSTRAINTS = {
   "person.firstname.maxlength" : 50,
   "person.lastname.maxlength" : 70
};

App.PersonController = Ember.ObjectController.extend({
  firstNameLength:CONSTRAINTS['person.firstname.maxlength'],
  lastNameLength:CONSTRAINTS['person.lastname.maxlength']
});

{{input type="text" value=firstname maxlength=firstNameLength}}

var CONSTRAINTS = {
    Person: {
        firstName: {
            maxlength: 50
        },
        lastName: {
            maxlength: 70
        },
    }
};

{{input type="text" value=firstname maxlength=CONSTRAINTS.Person.firstName.maxlength}}