title: TypeError: coercing to Unicode: need string or buffer, datetime.timedelta found ODOO10
body: <p>i'm using odoo 10 and i want to calculate arrival_time</p>
  <p>i Have two field an Float field datetime field , what i want is when the user put a value in that float field it will be add to that arrival time</p>
  <p>Here is my code:</p>
  <pre><code>from odoo import models
  from odoo import fields
  from odoo import api
  from datetime import datetime
  
  
  
  class TransportManagementLines(models.Model):
  _name = 'transport.management.lines'    
  _description = 'Transport Management Lines'    
  _rec_name = 'chauffeur'    
     
   chauffeur = fields.Many2one('transport.management.matricule',string='Nom Chauffeur', required=&quot;1&quot;,track_visibility='always')   
   starting_cities = fields.Many2one('transport.management.trajet',string='Trajet De Transport', required=&quot;1&quot;,track_visibility='onchange')   
   starting_time = fields.Datetime(string=&quot;Heure Départ&quot;, required=&quot;1&quot;,track_visibility='onchange')  
    arrival_time = fields.Datetime(string=&quot;Heure d'arriver&quot;, required=&quot;1&quot;, compute=&quot;set_arrival_time&quot;, store=True)
  
  
  
    @api.depends('starting_cities','starting_time')    
    def set_arrival_time(self):        
        for rec in self:           
        if rec.starting_time and rec.starting_cities:                
        rec.arrival_time = rec.starting_time + datetime.timedelta(seconds=(self.env['transport.management.trajet'].search([(&quot;nom_depart&quot;,&quot;in&quot;,rec.starting_cities.ids)],limit=1).travel_time  * 3600))                   
        else:                
              rec.arrival_time = False
  
  
  
  class TransportManagementTrajet(models.Model):    
  _name = 'transport.management.trajet'    
  _description = 'Transport Management Trajet'   
   _rec_name = 'nom_depart'          
   nom_depart = fields.Many2one('transport.management.config',string=&quot;Villes De Trajet&quot;, required=&quot;1&quot;)    
  travel_distance = fields.Integer(string='Nombre de KM', required=&quot;1&quot;)    
  travel_time = fields.Float(string=&quot;Durée de Trajet&quot;, required=&quot;1&quot;)```
  
  
  
  
  
  
  </code></pre>