Odoo many2many groupby example

1.py

    @api.model
    def read_group(self, domain, fields, groupby, offset=0, limit=None, orderby=False, lazy=True):
        if not '_context_' in groupby:
            return  super(Task, self).read_group(domain, fields, groupby, 
                            offset, limit, orderby, lazy)
        else:
            # Get the list of contexts
            res = []
            for c in self.env['nibbana.context'].search(
                                        [('create_uid','=', self.env.user.id)]):
                count = self.env['nibbana.task'].search_count([('context','in',c.id)])
                res.append({
                    '_context_': c.name,
                    '_context__count': count,
                    '__domain': expression.AND([domain, [('context','in', c.id)]])
                })
            return res