Odoo8.0: 'Null value in column' ERROR while writing a field to INVOICE

The error im receiving: ValueError: "null value in column "fleet_id" violates not-null constraint DETAIL: Failing row contains (2, null, null, 0.00, null, null, null, null, 1, 113, 2016-06-25 14:54:53.655, 1, null, null, null, 296, null, none, 2, null, draft, null, out_invoice, null, 1179, f, null, null, null, null, 2016-06-25 14:54:53.655, 1, 1, OR-003, null, OR-003, f, null, null, 1, null, null, null). " while evaluating u'wkf_invoice_create()' I am trying to write a field of vehicles (fleet_id) from MRP_REPAIR file to INVOICE in ODOO-8: The code i've added so far invoice = inv_obj.browse(cr, uid, invoice_id) inv_obj.write(cr, uid, [invoice_id],{'fleet_id': fleet_id}) these blocks are added to the function [action_invoice_create] in MRP_REPAIR.PY def action_invoice_create(self, cr, uid, ids, group=False, context=None): print "action_invoice_create---------------",ids, group, context repair = self.browse(cr, uid, ids[0]) res = super(mrp_repair, self).action_invoice_create(cr, uid, ids, group=group, context=context) inv_line_obj = self.pool.get('account.invoice.line') inv_obj = self.pool.get('account.invoice') for repair_id, invoice_id in res.iteritems(): if not invoice_id: continue invoice = inv_obj.browse(cr, uid, invoice_id) #activated by MSKE-25/06/2016 inv_obj.write(cr, uid, [invoice_id],{'fleet_id': fleet_id}) # added by MSKE-25/06/2016 for rsol in repair.order_line_ids: name = rsol.name or '' if rsol.product_id.property_account_income: account_id = rsol.product_id.property_account_income.id elif rsol.product_id.categ_id.property_account_income_categ: account_id = rsol.product_id.categ_id.property_account_income_categ.id else: raise osv.except_osv(_('Error!'), _('No account defined for product "%s".') % rsol.product_id.name) invoice_line_vals = { 'invoice_id': invoice_id, 'name': rsol.product_id.name, 'origin': rsol.repair_sale_order_line_id.name, 'account_id': account_id, #'quantity': rsol.product_uom_qty, 'invoice_line_tax_id': [(6, 0, [x.id for x in rsol.tax_id])], 'uos_id': rsol.product_id.uom_id.id, 'price_unit': rsol.price_unit, 'price_subtotal': rsol.product_uom_qty * rsol.price_unit, 'product_id': rsol.product_id and rsol.product_id.id or False } print "invoice_line_vals----------",invoice_line_vals invoice_line_id = inv_line_obj.create(cr, uid, invoice_line_vals) print "after line create----------------------------",invoice_line_id self.pool.get('account.invoice'). button_reset_taxes(cr, uid, invoice_id, context=context) return res I appreciate your help. edit - here is some of the code in the invoice.py: class account_invoice(models.Model): _inherit = "account.invoice" def _get_subtotal_word(self, cr, uid, ids, name, arg, context=None): res = {} for sale in self.browse(cr, uid, ids, context=context): res[sale.id] = self.convert_int_ot_word(sale.amount_total). upper() return res _columns = { 'subtotal_word': fields.function(_get_subtotal_word, type='char', string='subtotal_word'), 'reception_id': fields.many2one('garage.reception', 'Reception Report', readonly=True), # Procèss de reception 'fleet_id': fields.many2one('fleet.vehicle', 'VehicleMSKE', required=True, readonly=True), # Vehicle # Véhicules 'responsable_id': fields.many2one('res.users', 'Supervisor', readonly=True), # Responsable } Value for the fleet_id: 'fleet_id': this.fleet_id and this.fleet_id.id or None, located in the block (mrp_repair.py) def action_repair_done(self, cr, uid, ids, context=None): print "action_repair_done-------------------",ids, context res = {} move_obj = self.pool.get('stock.move') repair_line_obj = self.pool.get('mrp.repair.line') reception_obj = self.pool.get('garage.reception') for repair in self.browse(cr, uid, ids, context=context): move_ids = [] for move in repair.operations: repair_line_obj.write(cr, uid, [move.id], {'state': 'done'}, context=context) self.write(cr, uid, [repair.id], {'state': 'done'}, context=context) if repair.reception_id: print "repair.reception_id-----------",repair.reception_id reception_obj.write(cr, uid, [repair.reception_id.id], {'state': 'termine'}, context=context) if repair: this = repair # MSKE DEACTIVATED Bottom lines to alert when RO has no order lines # if (not this.sale_id) or (not this.sale_id.order_line): # raise osv.except_osv(_('User Error!'), # _("You must need add minimum single operation line for creation of invoice.")) reception_id = this.reception_id or None vals = { 'name': None, 'partner_id': this.partner_id and this.partner_id.id or None, 'reception_id': reception_id.id, 'partner_recived_from': reception_id and reception_id.partner_recived_from and reception_id. partner_recived_from.id or None, 'responsable_id': reception_id and reception_id.responsable_id and reception_id.responsable_id.id or None, 'fleet_id': this.fleet_id and this.fleet_id.id or None, 'kilometrage': reception_id.kilometrage, 'state': 'en_cours', 'datetime_livraison': time.strftime('%Y-%m-%d %H:%M:%S'), 'repair_id':this.id, } if this.service_requested_ids: vals.update({'service_done_ids' : [(6, 0, [service_id.id for service_id in this.service_requested_ids])] }) livraison_id = self.pool.get('garage.livraison'). create(cr, uid, vals, context=context) self.write(cr, uid, [this.id], {'livraison_id': livraison_id}, context=context) # ******************************** return True