Validating fields on the server side
We have created forms and fields, but we need to validate them in order to make sure that only the correct data goes through to the database and that errors are handled beforehand, rather than corrupting the database. These validations can also protect an application against cross-site scripting (XSS) and CSRF attacks. WTForms provides a whole lot of field types that, themselves, have validations written for them by default. Apart from these, there are a bunch of validators that can be used based on choice and need. In this recipe, we will use a few of them to understand the concept.
How to do it...
It is pretty easy to add validations to our WTForm fields. We just need to pass a validators parameter, which accepts a list of validators to be implemented. Each of the validators can have their own arguments, which enables us to control the validations to a great extent.
Let’s modify our ProductForm object in the models.py class to...