Hi, I want to add a unique constraint for a specific field but i have an error of ValidateError

Une erreur est apparue lors de la validation du/des champ(s) arch: Invalid XML for View Architecture!

my file.py

enter code here_columns = {
    'sequence': fields.integer('Sequence', help="Used to order the note stages"),
    'cne':fields.char('Matricule', size=64,required=True),

}

enter code here

_sql_constraints = [('student_state_name_unique','unique(cne)', 'State name already exists')]

myfile.xml

<?xml version="1.0" encoding="utf-8"?>

<openerp> <data>
<record model="ir.ui.view" id="view_school_student_form"> <field name="name">student.form</field> <field name="model">student</field> <field name="type">form</field> <field name="arch" type="xml"> <form string="Information Eleve" version="7.0"> <sheet> <notebook colspan="6">
<page string="Informations "> <separator string="Informations administratives" colspan="6"/> <group col="6" colspan="6"> <field name="sequence" widget="handler" invisible="1"/>