Flask WTForms SelectField, how to generate a choice parameter while changing data on a form?

Question

Flask

Good afternoon!
I'm new to Python and Flask (so sorry for the terms)
In short, I'll formulate the question:
I have a form with 4 fields and one button
like this
*———————————-
class RpaForm(FlaskForm):
title = StringField('Title', validators
area = SelectField('Automation Area', coerce=int)
cat = SelectField('Automation Category', coerce=int)
subcat = SelectField('Automation SubCategory', coerce=int)
submit = SubmitField( 'Save')
*————————————————
3 SelectFields from 3 different related tables
Roughly speaking, when selecting the AREA element, I need to filter the CAT table and show it in SelectField cat, well, from what I chose in cat, I need to filter the SUBCAT table and put the result in SelectField Subcat.

before sending data to the form (I don’t know what to call it) and render_template, of
course I do it all like this
*——————————————————-
form.area.choices = [(g .id, g.title) for g in Rpaarea.query.all ()]
form.cat.choices = [(c.id, c.title) for c in Rpacat.query.filter ( Rpacat.id_rpa == post. rpaarea ).all ()]
form.subcat.choices = [(b.id, b.title) for b in Rpasubcat.query.filter ( Rpasubcat.id_rpacat == post.rpacat ).all ()]
*——— ———————————————–
QUESTION: how can I change the filter conditions for CAT and so on when choosing an OTHER value in the AREA list.
Poke your nose ))))) I'm sure there is a simple and elegant solution.
Thank you in advance