I'm trying to expand the account.invoice model with functions and fields but seem to be unable because I keep getting this error.

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/odoo-8.0-py2.7.egg/openerp/http.py", line 544, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/usr/local/lib/python2.7/dist-packages/odoo-8.0-py2.7.egg/openerp/http.py", line 581, in dispatch
    result = self._call_function(**self.params)
  File "/usr/local/lib/python2.7/dist-packages/odoo-8.0-py2.7.egg/openerp/http.py", line 317, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/odoo-8.0-py2.7.egg/openerp/service/model.py", line 118, in wrapper
    return f(dbname, *args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/odoo-8.0-py2.7.egg/openerp/http.py", line 314, in checked_call
    return self.endpoint(*a, **kw)
  File "/usr/local/lib/python2.7/dist-packages/odoo-8.0-py2.7.egg/openerp/http.py", line 810, in __call__
    return self.method(*args, **kw)
  File "/usr/local/lib/python2.7/dist-packages/odoo-8.0-py2.7.egg/openerp/http.py", line 410, in response_wrap
    response = f(*args, **kw)
  File "/opt/odoo/odoo_server/addons/web/controllers/main.py", line 948, in call_button
    action = self._call_kw(model, method, args, {})
  File "/opt/odoo/odoo_server/addons/web/controllers/main.py", line 936, in _call_kw
    return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs)
AttributeError: 'account.invoice' object has no attribute 'init_fields'

I have this in my models.py:

class invoice_fields(models.Model):  

    _inherit = "account.invoice";

    def init_fields(self):

        self.ExtendModel();

.....

I try to call that function from the templates.xml file with a button using type="object" and name="init_fields".

As you can see I have inherited account.invoice but for some reason my function isn't working. What could be the problem?

I'm using Odoo 8.0.