Removing required constraint on link fails to create migration

[jackfischer]

• EdgeDB Version: EdgeDB CLI 2.0.5+53f9c9b, EdgeDB 2.1+9c4bc9f
• OS Version: macOS 12.6

(Follow up from discord)

Steps to Reproduce:

1. Drop a required constraint on a link, making it optional.

-        required link subject -> SignalSubject;
+        link subject -> SignalSubject;

$  edgedb migration create
did you make link 'subject' of object type 'default::Signal' optional? [y,n,l,c,b,s,q,?]
> l
The following DDL statements will be applied:
    ALTER TYPE default::Signal {
        ALTER LINK subject {
            RESET OPTIONALITY;
        };
    };
did you make link 'subject' of object type 'default::Signal' optional? [y,n,l,c,b,s,q,?]
> y
edgedb error: EdgeDB could not resolve migration with the provided answers. Please retry with different answers.

Schema:

[jackfischer]

This appears more complicated than the above; just made the same change on another required link successfully:

  ALTER TYPE default::StrategyNode {
      ALTER LINK documentVersion {
          RESET OPTIONALITY;
      };
  };

Both required links were originally created the same way. From the original migration,

CREATE TYPE default::StrategyNode EXTENDING default::SignalSubject {
        CREATE REQUIRED LINK documentVersion -> default::DocumentVersion;
...
CREATE TYPE default::Signal {
        CREATE REQUIRED LINK subject -> default::SignalSubject;

The only difference I see is that the target of the failing link is abstract.

[msullivan]

I was unable to reproduce this issue while doing the obvious thing, so there must be something more subtle going on that is triggering this for you.
Do you have a more complete example of a schema where it is failing? Are there a lot of migrations that have been run already?

[jackfischer]

Attempted to reproduce more cleanly here.

Unfortunately my theory it was due to SignalSubject being abstract is wrong
jackfischer/edgedb-4371@4bee1e7

[jackfischer]

Found it!!

Making the following change to remove the computed backlink for this relationship allowed the removal of required.

   abstract type SignalSubject {
-        multi link signals := .<subject[is Signal];
+        # multi link signals := .<subject[is Signal];
     }

[msullivan]

Great! Do you have a small reproducing case you can share, then?

[jackfischer]

Yup live here - https://github.com/jackfischer/edgedb-4371, default.esdl is edited so it's ready for the failing edgedb migration create.