[Python] how to define a foreign key with pony entities

I would like to define a foreign key with pony entities.

If I understand it right, there is no need to define a primary key as long as it is named id. so the primary key of Jobs is id, and I want to define a foreign key job_id on Recipe which is related to id of Jobs. I tried Required(Jobs.id) but this gives a type error. do you have any hint on how to do it? thank you

class Jobs(db.Entity):
path = Required(str)
date = Required(str)

class Recipe(db.Entity):
job_id = Required(int) # must be foreign
path = OptionalField(str)
date = OptionalField(str)

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