In V8, you don't need to provide _columns={}. Go through this for clear description.

Base:
class account_invoice(models.Model):
    _name = 'account.invoice'
    name = fields.Char(string='Reference/Description') #Base field

Your module:
class account_invoice(models.Model):
    _inherit = 'account.invoice'
    description = fields.Char(string='Your Description') #Inherited Field

Hi Mohammed,

you can add the custom field by using inheritance.By inheritance you can add or remove or update any columns in the table.

for example:

In the .py file:

class account_invoice(orm.Model):
    _inherit="account.invoice"
    _columns={
            'your_field':fields.char('New Field')
}

in the xml view:

        <record id="account_invoice_form_inherit" model="ir.ui.view">
            <field name="name">account.invoice.form.example</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_form"/>
                <field name="date_invoice" position="after">

                     <field name="your_field" />
                </field>
            </field>
        </record>

Hi Vasanth

i tried the above method, its not the way to accomplish it in account.invoice.

if you go through the account_invoice, you will find there is no _columns present and

"( _columns={
            'your_field':fields.char('New Field')
}) "   

this method is not followed in it.

its something else

anyways thanks for your answer