I have defined this functional field so that I can get values from one2many field and display them in tree view:

    def get_product_brands(self, cr, uid, ids, fields, arg, context):

        context = context or {}

        res={}

        for record in self.browse(cr, uid, ids, context=None):

            brands = ''

            for val in record.application_data_product_template_ids

                if val.brand.name:

                    brands += "%s " % (val.brand, )

                print brands

            res[record.id] = brands and brands[:-1] or ""

        return res

my field defintion:

'brands' : fields.function(get_product_brands, method=True, string="Product brands", type='char')

But I'm getting this error:

File "/home/odoo/addons/bal_dev/balerce_dev.py", line 209, in get_product_brands_2

for record in self.browse(cr, uid, ids, context=None).application_data_product_template_ids:

File "/home/odoo/openerp/fields.py", line 755, in __get__

record.ensure_one()

File "/home/odoo/openerp/models.py", line 5219, in ensure_one

raise except_orm("ValueError", "Expected singleton: %s" % self)

except_orm: ('ValueError', 'Expected singleton: product.template(8, 4, 5, 9, 2, 3, 7)')