Title says it all: I have 2 models, foo.bar and foo.bar.line with a one2many relation. foo.bar.line has a text field on which it is useful to perform a "group by" : how can I show the foo.bar.lines within the foo.bar form view as a tree view with a group_by?

I've tried:

            <field name="line_ids"
                   colspan='4' context="{'group_by': 'group_field }">
              <tree....> </tree>
          </field>

with little success.