I created a sequence via user interface : Configuration/Technical/Sequences and identifiers/ Sequences

I also created its code so as so to call it, on the python method on : Configuration/technical/Sequence Code

The model concerned is account.invoice , I want that when validating the invoice, it switches to state open, and generate a sequence  number, for that : 

1.  I overrided the type of validate invoice button to object ( it was workflow )

2.  I used invoice_validate as a button method , and overrided it.

3.  I overrided the number field, it was related, I made it simple char field.

and then I used the sequence, after the code for more explaination.

.py :  

class AccountInvoice(orm.Model):

    _inherit = 'account.invoice'
    _name = 'account.invoice'
    _colmuns = {
        'number': fields.char(size=64, readonly=True),
    }

    def invoice_validate(self, cr, uid, ids, context=None):

        num_facture = self.pool.get('ir.sequence').get(cr, uid,'Invoice_number')

        self.write(cr, uid, ids[0], {'state':'open','number': num_facture,}, context=context)
        return True

.xml :

        <record id="invoice_form_custom" model="ir.ui.view">
            <field name="name">invoice.form.custom</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_form"/>
            <field name="arch" type="xml">
                <xpath expr="//form/header/button[@name='invoice_open']" position="replace">
                    <button name="invoice_validate" states="draft" string="Valider" type="object" class="oe_highlight"/>
                </xpath>
            </field>
        </record>

The issue is that the sequence is not displayed on the field "number" I overrided, should I add "number" on the xml view also ?

Thanks