line_mission = fields.Many2many('hr.employee', string='Employés', help="Employee sent on mission",
                            copy=True, auto_join=True,readonly=True, states={'draft': [('readonly', False)]})

@api.multi
    @api.constrains('mission_start_date', 'mission_end_date')
    @api.depends('line_mission.name')
    def _check_date(self):
        for mission in self:
            employee = self.env['hr.employee'].search_count([('name', '=', self.id)])
            domain = [
                ('mission_start_date', '<=', mission.mission_end_date),
                ('mission_end_date', '>=', mission.mission_start_date),
                ('id', '!=', mission.id),
                ('state', 'not in', ['end_mission']),
            ]
            nmissions = self.search_count(domain)
            if employee in mission.line_mission.name and nmissions:
                raise UserError(_('Vous ne pouvez pas avoir 2 missions qui se chevauchent le même jour!'))