this.model in quote-data-group-list does not have relate field from record.php

Hello,

I've been struggling to figure this one out and wondered if anyone had any ideas. We have a requirement to make a relationship between products and Tax Rates, selectable in the 7.9+ Sidecar quote-data-group-list.php view.

Here's a brief outline of what I've done:

I have this vardef in custom/Extension/modules/Products/Ext/Vardefs/file.php

$dictionary['Product']['fields']['vat_rate'] = array(
     'name' => 'vat_rate',
     'vname' => 'LBL_LIST_VAT_RATE',
     'type' => 'id',
     'required'=>false,
     'do_report'=>false,
     'reportable'=>false,
);
$dictionary['Product']['fields']['vat_rate_name'] = array(
    'name' => 'vat_rate_name',
    'rname' => 'name',
    'id_name' => 'vat_rate',
    'join_name' => 'taxrates',
    'type' => 'relate',
    'link' => 'taxrates',
    'table' => 'taxrates',
    'isnull' => 'true',
    'module' => 'TaxRates',
    'dbType' => 'varchar',
    'len' => '255',
    'vname' => 'LBL_LIST_VAT_RATE',
    'source' => 'non-db',
    'comment' => 'Vat Rate Name',
    'massupdate' => false,
);

I've copied quote-data-group-list.php to custom from modules/Products/clients/base/views/quote-data-group-list.php and added this just before the total_amount array:

array(
    'name' => 'vat_rate_name',
    'type' => 'quote-vat-relate',
    'initial_filter' => 'active_taxrates',
    'filter_populate' => array(
        'module' => array('TaxRates'),
    ),
    'related_fields' => array(
        'vat_rate_name',
        'vat_rate',
        'taxrates'
    ),
),

You'll see I have a custom field type called quote-vat-relate, which I've mocked from the product template selector, extending the BaseRelateField type, located at custom/modules/Products/clients/base/fields/quote-vat-relate.js

({
    extendsFrom: 'BaseRelateField',

    format: function(value) {
        var idList;
        value = value || this.model.get('vat_rate_name');

        this._super('format', [value]);

        if (value) {
            idList =  this.model.get('vat_rate');
            if (_.isArray(value)) {
                this.formattedIds = idList.join(this._separator);
            } else {
                this.formattedIds = idList;
            }

            if (_.isEmpty(this.formattedIds)) {
                this.formattedIds = value;
            }
        }

        return value;
    },
})

This all seems to be going well as when I edit the items, I can select a VAT code, and I can confirm it stores in the database. Great!

Navigate, and come back, and it's disappeared!

When I dug into this, I found that when the format() function is initially invoked, this.model doesn't contain my field. Even stranger is that when I move the product lines around, it invokes format() again, and the value suddenly displays! When I looking at the value of this.model now, it contains all of the attributes, including vat_rate_name now!

So it's seems to be down to an initial display issue. Can I ensure that this field is included in this.model from the start? Is this a bug?

Any suggestions greatly appreciated.