How to get a blank string for a new model instance's idProperty?

I have the following model which, under Ext JS 4.x, would result in a blank string (or null, I think) for the 'code' field when creating a new model instance:

Code:

Ext.define('App.model.TrainingCode', {
    extend: 'Ext.data.Model',
    requires: [
        'Ext.data.field.String'
    ],
    idProperty: 'code',
    fields: [
        {
            type: 'string',
            name: 'code'
        },
        {
            type: 'string',
            name: 'classification'
        }
    ]
});

How do I achieve this behaviour in Sencha Architect 3.1 with Ext JS 5.0.1? It is linked to a REST store so I can't set the blank string myself when I create the record (which I'm stuffing into a grid and using a row editor), because doing so results in a PUT and not a POST. Setting the idProperty value on creating a new model makes it no longer a phantom record, so I'm in a nasty catch-22 with these new auto-generated rubbish values for idProperty fields. I just want null or a blank string.



How to get a blank string for a new model instance's idProperty?