What is the correct way to organize validation in Rails?

Question

Ruby on Rails

Good afternoon!
I am developing an application on Ruby on Rails, I ran into a problem: how to correctly check the field before executing the method?
For example, there is an Invitation model (invitation) and it has an accept method . Before calling the accept method , it would be necessary to check the data that the user sends against the data in the database. In a bad result, terminate the method and return an error that the controller can render.
It was possible to nagovnocode, but I want to do it in rails-way