In this recipe, we will manually create a record of the library.return.wizard model. We want the onchange method to compute the returned books for us. To do this, you need to perform the following steps:

1. Create the return_this_books method in the library.book model:

    @api.multi 
    def return_all_books(self): 
        self.ensure_one()

2. Get an empty recordset for library.return.wizard:

    wizard = self.env['library.return.wizard']

3. Prepare the values to create a new wizard record. Here, we will use a current user's partner ID as the borrower_id, but if you want to put this button on the res.parnter model, you can use self.id:

    values = {
            'borrower_id': self.env.user.partner_id.id,
        }

4. Retrieve the onchange specifications for the wizard:

specs = wizard._onchange_spec() 

5. Get the results of the onchange method:

updates = wizard.onchange(values, ['borrower_id'], specs)

6. Merge these results with the values of the new wizard:

value = updates.get('value', {})
for name, val in value.items():
    if isinstance(val, tuple):
        value[name] = val[0]
values.update(value)

7. Create the wizard:

wiz = wizard.create(values)
return wiz.sudo().books_returns()