Calculation Computed Properties in Models Ember

I am trying to achieve a calculation with computed properties between a value inserted in an Input and a value selected in the option value in order to see the data stored

You can see here

This is my code in the models

App.Invoice = DS.Model.extend({
  title         : DS.attr('string'),
  transactions  : DS.hasMany('transaction', { async:true})
});

App.Transaction = DS.Model.extend({
  quantity: DS.attr('string'),
  fare: DS.attr('string'),
  total: DS.attr('string'),
  isChecked: DS.attr('boolean')
});

App.Total = Ember.Object.extend({
  quantity: null,
  fare: null,
  total: function(){
    return this.get('quantity')*this.get('fare');
  }.property('quantity', 'fare')
}); 

And then in my template

<tr>
{{#each model.transactions}}
  <td>{{name}} {{input type="checkbox" value="isChecked" checked=isChecked}} {{input type="number" value=quantity}}{{view Ember.Select prompt="Tariffa" value=fare content=controller.selectContentTariffa optionValuePath="content.value" optionLabelPath="content.label"}} {{input type="number" value=total}}</td>
{{/each}}
</tr>

Unfortunately I can not achieve it and I don’t know what I am doing wrong, however the record created for the fare and quantity are displayed correctly but not for the total, because i can not calculate it in this way

See the img

In comparison to that, I achieved when I made this multiplication in the controllers (see here) but obviously in this way I can not create the data record for the transaction elements (fare, quantity and total)

See the img

So what ‘s the way to achieve this operation in order to create the record data for the quantity, fare and total?