24 cases for `$addFields`/`$set`, `$unset`, `$replaceRoot`, `$unwind` and
`$project`'s computed fields, recorded from mongod 8.3.7 before any of them is
written. The file starts at 0 pass / 24 fail, which is the honest number for
five stages this server does not have.
Eight things the recording settled, none of them guessable from the manual:
$addFields whose expression is missing the field is not added at all
$addFields: {"n.z": 1} sets the nested path, keeps siblings
$replaceRoot of missing or non-document error 40228
$unwind of an empty array or missing the document is dropped
$unwind of a non-array the document is kept whole
$unwind path without a $ error 28818
includeArrayIndex 0-based
$project: {n: {x: 1}} narrows it; a document without
`n` keeps only `_id`
That last one is the `query.project` gap recorded during M2 as broken rather
than unimplemented -- a nested inclusion currently reads as falsy and returns
the whole document minus the field. It now has a measured expectation to be
fixed against, in the corpus, rather than a note in a commit message.
Worth stating before the implementation: the design review expected Tier 2 to
need a per-stage iterator, on the grounds that `$unwind` is 1->N and "there is
no way to express that in a window over the input". That was true of the window
as it stood, and stopped being true when `$project` was made to rebuild the
stream -- a stage that rebuilds can emit any number of documents it likes. So
Tier 2 looks like five stages rather than a rewrite. The corpus is what will
say whether that holds.
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