How can I create a foreign key constraint using Yesod/Persistent?

Question

Is there a way to create a foreign key constraint using Persistent's schema syntax with the Postgres backend? Or do I need to do this manually with SQL? Specifically, an ON DELETE CASCADE relationship such that when a HackDay is deleted, all of its child Projects are deleted:

HackDay
    title Text
    created UTCTime default=now()
    votingClosed Bool default=false
    deriving Show
Project
    hackday HackDayId
    title Text
    creators Text
    votes Int default=0
    created UTCTime default=now()
    deriving Show

Answer 1

Persistent does not currently have any built-in support for triggers, though it's something we've been wanting to add (simply lacking manpower). For now, you'll have to add the trigger manually.