Undefined accessing jquery selector with object property
===

I have an issue using an object property as jquery selector


    define(["models/security/user", 'text!templates/security/registration.html'], function(SecurityUserModel, Template){
    
        var SecurityRegistrationView;
    
        SecurityRegistrationView = Backbone.View.extend({
            initialize: function(){
                // setting model
                this.model = new SecurityUserModel();
    
                // binding validation
                Backbone.Validation.bind(this);
                this.model.bind("validated:valid", this.valid);
                this.model.bind("validated:invalid", this.invalid);
    
                this.render();
            },
            form: {
                "username": "#_user_username"
                , "email": "#_user_email"
                , "password": "#_user_password"
            },
            render: function(){
                $(this.el).append(Template);
            },
            events: {
                "submit form": "submit"
            },
            submit: function(e){
                e.preventDefault();
    
                this.model.set("username", $(this.form.username).val());
                this.model.set("password", $(this.form.email).val());
                this.model.set("email", $(this.form.password).val());
                this.model.validate();
    
                if (this.model.isValid) {
                    this.model.save();
                }
            },
            valid: function(model, attrs){
                console.log(attrs[0]);
                $(this.form[attrs[0]]).parent("div.control-group").addClass("success");
            },
            invalid: function(model, attrs){
                console.log(attrs[0]);
                $(this.form[attrs[0]]).parent("div.control-group").addClass("error");
            }
        });
    
        return SecurityRegistrationView;
    });

Console output in chromium:

    username registration.js:45
    Uncaught TypeError: Cannot read property 'username' of undefined registration.js:46
    password registration.js:45
    Uncaught TypeError: Cannot read property 'password' of undefined registration.js:46
    email registration.js:45
    Uncaught TypeError: Cannot read property 'email' of undefined registration.js:46
    username registration.js:45
    Uncaught TypeError: Cannot read property 'username' of undefined registration.js:46



    $(this.form.username).val(); // works
    $(this.form[someVar]); // does not work