The error also gives a hint

"

HINT:  You will need to rewrite or cast the expression.

"

here's my python code:

class AccountInvoice(models.Model):
    _inherit = 'account.invoice'

    start_date = fields.Date()
    end_date = fields.Date()
    total_months = fields.Integer(compute='_total_months')
    deferred_revenue = fields.Boolean()

    @api.multi
    def _total_months(self):
        if self.start_date and self.end_date:
            for rec in self:
                date1 = dt.strptime(str(rec.start_date), '%Y-%m-%d')
                date2 = dt.strptime(str(rec.end_date), '%Y-%m-%d')
                r = rd(date2, date1)
                rec.total_months = r.months

    @api.multi
    def action_invoice_open(self):
        res = super(AccountInvoice, self).action_invoice_open()
        obj = self.env['account.asset.category'].browse([('method_number', '=', self.total_months)]).id
        if self.deferred_revenue == True:
            self.env['account.asset.asset'].create({
                'name' : self.name or 'kkkkk',
                'category_id' : obj,
                'date' : self.date,
                'value' : self.amount_total,
                'partner_id' : self.partner_id.id,
                'invoice_id' : self.id,


            })
        return res

i want 'category_id' to get  a specified account from model 'account.asset.category' on the basis of total_months. 

In model  'account.asset.category' method_number is defined. for example if total_months = 5 then the category_id should be 'cat 5' from 'account.asset.category' model. i've tried different solutions. hope I have explained my problem.