Fix bug with trap() around lists/connections

[benjie]

Consider a plan that wants to skip unnecessary work, such as:

function myPlan($parent, fieldArgs) {
  const { $ids } = fieldArgs;
  const $records = myResource.find();
  $records.where(sql => sql`${$records}.id = ANY(${$records.placeholder(
    inhibitOnEmpty($ids),
    listOfCodec(TYPES.int)
  )})`);
  return trap($records, TRAP_INHIBITED, { valueForInhibited: "EMPTY_LIST" });
}

$records, being a PgSelectStep, is a .items() capable step. It actually represents at runtime an object, with the items list inside of it. But now we're returning trap() (which is a __FlagStep), which did not support .items() - so when the dependency is not inhibited, we just return the PgSelectStep value verbatim... and that's not a list, so we return null! trap() breaks our list!

The fix is straightforward: have __FlagStep implement the $step.items() pattern, re-wrapping the resulting access to the underlying step with the same flag.

Connections

For connections things are much more complex. Connections need to not just represent an empty list but potentially deal with aggregates and other complexities. We can't know which values to use for aggregates (string_agg over an empty list may be "", array_agg maybe [], count() may be 0, etc) so we can't sensibly create a default. Having the user trap(..., valueForInhibit: "EMPTY_LIST") doesn't make sense for a connection because it expects an object with more details: cursors, etc. Basically... I timed out figuring out a good way to make this work. In the end I decided to forbid trap() with connections; instead we should teach the underlying steps how to avoid doing work if it's unnecessary without leaning into the inhibit system. If you use inhibit in a field that returns a connection, you may well end up with that field returning null rather than an empty connection. Recommendation: don't inhibit with connections for now.

[changeset-bot]

🦋 Changeset detected

Latest commit: 3101d99

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@dataplan/pg	Patch
grafast	Patch
postgraphile	Patch

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