gistfile1.js

GX.itemController = SC.ObjectController.create({
  invoiceVatBinding: 'GX.invoiceForItemController.hasVAT',
  
  hasVAT: function(){
    return this.get('invoiceVat');
  }.property('invoiceVat')
});

GX.invoiceForItemController = SC.ObjectController.create({
  contentBinding: "GX.itemController.invoice"
});

//===

GX.Item = SC.Record.extend({

  invoice: SC.Record.toOne("GX.Invoice", { inverse: "item" }), 
  
  invoiceDidChange: function () {
    this.notifyPropertyChange('invoice');
  }.observes('invoiceHasVAT'),

  
  invoiceHasVAT: function() {
    return this.getPath('invoice.hasVAT');
  }.property('invoice').cacheable(), // replacing this by .property('invoice.hasVAT').cacheable() and removing the observer above also throw an error

  ...

});