It set a variable that the emit applied once. Three consequences, all measured
on a live server before this changed rather than read off the code:
- only the *last* `$project` in a pipeline had any effect;
- a `$match` after one still matched on the field it had removed;
- `{y: {$literal: 5}}` read as falsy, which flipped the whole projection into
its exclusion branch and returned every document minus `y`, where mongod
adds a computed `y`.
Now it transforms the stream where it stands, materializing into the same tree
form `$group` already produced. `$match`, `$sort` and `$group` after it read
what it left, which is what "stage" means.
What it cannot do is refused rather than mis-read. A computed field needs the
expression evaluator M2.5 brings, and a nested spec (`{a: {b: 1}}`) needs a
narrowing `query.project` does not do -- both were falsy, and falsy is the
answer that quietly returned the whole document. Codes read off mongod 8.3.7,
and the replies now match it on code, codeName and message text for the empty,
the mixed and the computed forms:
projection must have at least one field 51272 Location51272
cannot mix inclusion and exclusion 31254 Location31254
unknown expression in $project 31325 Location31325
Recorded, not fixed here: `query.project` is shared with `find`'s `projection`,
where a nested inclusion has the same falsy reading and the same wrong answer.
That is a real gap rather than an unimplemented feature -- narrowing is
something this projection should do -- so it is its own fix, not a refusal, and
it belongs with whichever milestone takes `find`'s projection seriously.
Scorecard unchanged again at 194/97/196. The corpus has no `$project` stage
tests either; A6 said so, and this is the second commit to confirm it.
Mutations: drop the refusals and the empty projection answers `ok: 1`; put the
projection back on the emit and the `$match`-after-`$project` case goes red.
190/190 unit tests in ReleaseFast and ReleaseSafe, 83/83 fuzz, e2e 49, e2e2
concurrent, e2e3 16, e2e4 17, e2e6 72, e2e7 86.
61ce9805c7