The monetary field needs a complementary currency field to store the currency for the amounts.

The my_module already has a models/library_book.py defining a basic model. We will edit this to add the required fields:

1. Add the field to store the currency that is to be used:

class LibraryBook(models.Model): 
    # ... 
    currency_id = fields.Many2one( 
        'res.currency', string='Currency') 

2. Add the monetary field to store our amount:

class LibraryBook(models.Model): 
    # ... 
    retail_price = fields.Monetary( 
        'Retail Price', 
        # optional: currency_field='currency_id', 
        ) 

Now, upgrade the addon module, and the new fields should be available in the Model. They won't be visible in views until they are added to them, but we can confirm their addition by inspecting the Model fields in Settings | Technical | Database Structure | Models.