仅在保存客户后显示标题按钮的条件

<xpath expr="//form/*" position="before">
                    <header>
                        <button name="offer_letter" string="Offer Letter" type="object" class="oe_highlight"
                                attrs="{'invisible':[('what condition?')]}"/>
                    </header>
                </xpath>

模型:

class res_partner(models.Model):
    _inherit = 'res.partner'

    baf = fields.Boolean("Application Form", help="Specify customer who bought application form")

    @api.multi
    def offer_letter_method(self):
        return self.env['report'].get_action(self, 'sales_custom.offer_letter_view')

<record model="ir.ui.view" id="customer_custom_form_view">
            <field name="name">customer.custom</field>
            <field name="model">res.partner</field>
            <field name="inherit_id" ref="base.view_partner_form"/>
            <field name="arch" type="xml">
                <xpath expr="//form/*" position="before">
                    <header>
                        <button name="offer_letter" string="Offer Letter" type="object" class="oe_highlight"
                                attrs="{'invisible':[('what condition?')]}"/>
                    </header>
                </xpath>
                <xpath expr="//field[@name='name']" position="after">
                    <field name="baf"/>
                    <label for="baf"/>
                </xpath>
            </field>
        </record>

请协助。