Hello, I have Odoo 8.

I need to add a "group by" button in a report. I know that the XML code is like this:

<filter name="groupby_name" context="{'group_by': 'field''}" string="Description" />

But I have a model A on where I do the group by and this model has a many2one relation with a model B. The attribute of the many2one relation is named "rel". If I need to group by over any field of model B, how can I do? If I write

<filter name="groupby_name" context="{'group_by': 'rel.name_field''}" string="Description" />

But when I press the button to grupo by I get: TypeError: this.fields[field] is undefined

How can I do this groupby? Thanks!