There’s a difference between a Model, which is your internal representation of your database, and a Form, which is your internal representation of a (or part of a) page.
What you’re looking to do here has nothing to do with your model. It’s associated with creating a widget on your page. In this particular case, you’re going to be creating a select widget for your field on the form, supplying choices that are dynamically created from your database.
This topic has been covered previously on the forum here - see Populate choice field from model with conditional query Django