Why can't links have required properties?

[CodesInChaos]

What's the technical reason for that limitation?

Attempting to work around the limitation with constraint expression on (exists(__subject__@index)); fails with "I
nternalServerError: cannot represent operator EXISTS in Postgres". (I didn't expect it to work, since if this worked you probably wouldn't have this limitation in the first place)

[elprans]

Support for required link properties was removed some time ago because of the difficulties in implementing them reasonably at the time (both internally and semantically). We might consider restoring support.

cannot represent operator EXISTS in Postgres

That's actually just missing support for EXISTS in simple expression transformer. Should be easy to fix.

[adriangb]

I'd be +1 on this feature being implemented.

[rostero1]

Is the constraint expression the recommended workaround?

[rostero1]

Example:

module default {
  type Address {
    name: str;
  }

  abstract link coordinates {
    coordinates: tuple<lng: float32, lat: float32>;
  }

  type Place {
    address: Address {
      extending coordinates;
      constraint expression on (exists(__subject__@coordinates)) {
        errmessage := '{__subject__} requires coordinates.';
      }
    }
  }
}

[elprans]

Yes, that's right. In fact, I think we should reconsider allowing linkprops to be required. Opened #5571 to track.

[aqc-carlodri]

This workaround seems not to work for me, it gives this error:

error: cannot use aggregate functions or operators in a non-aggregating constraint

do you have any other suggestion to obtain a required link property?