How to create backlinks in DQL for graphql

Schema that links Events to Contacts:

type Contact {
  id: ID
  hasEvents: [Event] @hasInverse(field: at)
  ...
}

type Event {
  id: ID
  at: Contact @hasInverse(field: hasEvents)
  ...
} 

I imported the data with:

{
  set {
    _:contact_1 <dgraph.type> "Contact" .
    ...
    _:event_1 <dgraph.type> "Event" .
    _:event_1 <Event.at> _:contact_1 .
    ...
  }
}

As you see, I forgot to add the reverse of _:contact_1 <Contact.hasEvents> _:event_1 . In my import script. Is there a way to fix this after the fact and somehow create the reverse for every Event.at edge and map it to Contact.hasEvents edge?

I was thinking of an upsert like:

upsert {
  query {
    event as var(func: has(Event.at)){
      Event.at {
        contact as uid
      }
    }
  }
  mutate {
    set {
      contact <Contact.hasEvent> event .
    }
  }
}

But this will set every event for every contact, and does not produce the desired results.

Is this possible without FOREACH func in GraphQL +- (loops in Bulk Upsert)