I try to override a basic workflow of the account.invoice model. I made a module with some a view that add two button, type 'action' calling two methods of a model that inherit of account.invoice. When I presse any of those two buttons odoo rise an error meaning the action isnt defined in account.invoice. I expected to move forward through the pipeline.

here is my model:

from odoo import api,fields,models, _

class Account_invoice_custom(models.Model):
   _inherit ='account.invoice'
   state = fields.Selection(selection_add([('brocker', 'Brocker'),('furnisher','Furnisher')]))
   
   @api.multi
   def action_invoice_brocker(self):
    self.state = 'brocker'
    return true
   
   @api.multi
   def action_invoice_furnisher(self):
    self.state = 'furnisher'
    return true

Here is my view:

<odoo>

  <record model="ir.ui.view" id="view_invoice_form">
    <field name="name">account.invoice.form.custom</field>
    <field name="inherit_id" ref="account.invoice_form"/>
    <field name="model">account.invoice</field>
    <field name="arch" type="xml">
      <xpath expr="/form/header/field" position="before">
        <button name="action_invoice_brocker" states="paid" string="brocker" type="object" class="oe_highlight o_invoice_validate" groups="account.group_account_invoice"/>
      </xpath>
      <xpath expr="/form/header/field" position="before">
        <button name="action_invoice_furnisher" states="paid" string="furnisher" type="object" class="oe_highlight o_invoice_validate" groups="account.group_account_invoice"/>
      </xpath>
      <xpath expr="/form/header/field[@name='state']" position="replace">
        <field name="state" widget="statusbar"  statusbar_visible="draft,open,paid,brocker,furnisher" />
      </xpath>
    </field>
  </record>

</odoo>