Hi,

I have an extended model of `res.partner` with a field named `x_subsidiary_of`.

When I am in the search view of customer invoices I would like to group invoices by this `x_subsidiary_of` field.

I've tried to modify the search view adding the following line :

    <filter string="Partner" icon="terp-partner" domain="[]" context="{'group_by':'partner_id.x_subsidiary_of'}"/>

But I get this error :

    AssertionError: Fields in 'groupby' must be regular database-persisted fields (no function or related fields), or function fields with store=True

How could I achieve this ?