# The aggregation corpus

`mongodb/specifications` has no aggregation suite. The thirteen
`aggregate-*.json` files this project runs come from `crud` and test the
aggregate *command* — cursors, read concern, the write stages, collation,
`let`. They touch stages barely and expressions not at all: `$lookup`,
`$unwind`, `$facet`, `$addFields` and `$replaceRoot` appear nowhere in the
pinned corpus. That is PLAN amendment A6, and this directory is its
consequence: M2.5 has to bring its own gate.

## The one rule

**Inputs are authored here; expectations are measured against a real mongod.**

A corpus we write is a corpus that can encode our own bugs as expectations, and
it would then agree with us forever. So `sources/*.json` holds documents and
pipelines and nothing else, and `record.js` asks mongod 8.3.7 what each pipeline
answers. It is the same discipline that corrected three assumptions in M1's
session work and every error code in M2 — the alternative, in both cases, would
have shipped.

## Running it

```sh
node tests/spec/run.js --suite-dir tests/spec/aggregate
```

The same runner as the crud corpus, pointed elsewhere. Sharing it is the point:
the entity model, the matchers, the skip accounting and `expectEvents` come for
free, and a second runner would drift from the first exactly where it mattered.

`--scorecard` is refused with `--suite-dir`: `tests/spec/scorecard.txt` is the
crud corpus's record and the milestones are compared against it.

## Re-recording

```sh
mongod --port 27099 --dbpath /tmp/mongo-corpus &
node tests/spec/aggregate/record.js --mongod-port 27099
```

Writes `<name>.json` for every `sources/<name>.json`. The generated files are
committed: they *are* the corpus, and regenerating them is how a disagreement
with mongod gets re-measured rather than argued about.

Two things to know when adding cases:

- **End a `$group` pipeline with a `$sort`.** Group output order is unspecified,
  and a case that depended on it would fail for the wrong reason on either
  server.
- **Errors record the code, not the message.** Message text is mongod's to
  change between releases; a corpus that pinned it would break for the wrong
  reason.

Leave out any case whose answer depends on a server newer than the 4.4 this
server reports — recording it from mongod 8.x and judging it against a 4.4
answer measures the version gap, not the engine.

## Where it stands

Recorded against mongod 8.3.7. At the M2 tip it read 9 pass / 10 fail; with the
accumulators in:

```
group-accumulators.json   18 pass    1 fail   0 skip
expressions.json           1 pass   26 fail   0 skip
```

`group-accumulators` found its first real disagreement on the way to 18:
`$avg` over a group with no numeric value is `null`, not `0`, and a divisor
that counted documents rather than numbers would have passed every test
anybody would think to write by hand.

`expressions.json` is the next tier's spec, recorded and not yet implemented --
its one pass is the unknown-operator refusal M2 already answers correctly.
Expressions are exercised through `$group`, because `_id` and the accumulator
arguments are the only expression positions that exist until `$addFields` and
`$project`'s computed fields land.

What recording it settled, none of which is guessable:

| | mongod |
|---|---|
| `$add` over a missing field or null | `null`, not an error and not `0` |
| `$add` over a string | error 7157723 |
| `$divide` by zero | error 4848401 |
| `$mod` of -5 by 4 | `-1` — the dividend's sign |
| `$lt` of a number and a string | `true` — canonical type order |
| `$and` over `-5` | truthy |
| `$not` of a missing field | `true` |
| `$switch` with no branch and no default | error 40069 |
| two operators in one expression document | error 15983, *not* `$group`'s 40238 |
| `$subtract` with one operand | error 16020 |