Chi Fujii

### What should be done? We have test data providers `scalars.go` and `composites.go` used for compat tests. But test filters we add for each operators are up to our creativity....

Validate `$` prefix in path construction in `newPath`

4

### What should be done? Path should not have `$` prefix. In this issue add validation of `$` prefix in `newPath`. https://github.com/FerretDB/FerretDB/blob/658b8e23a9337324d3f6cc266ffab10ac4e9083b/internal/types/path.go#L107-L109 When `$` prefix validation is added, query projection...

Remove localhost exception

1

# Description Closes #4236. ## Readiness checklist - [ ] I added/updated unit tests (and they pass). - [ ] I added/updated integration/compatibility tests (and they pass). - [ ]...

### What should be done? https://www.mongodb.com/docs/manual/reference/operator/aggregation/lastN/#mongodb-group-grp.-lastN ### Where? https://github.com/FerretDB/FerretDB/tree/main/internal/handlers/common/aggregations/operators/accumulators ### Definition of Done - all handlers updated; - integration/compatibility tests added/updated; - spot refactorings done; - user documentation updated or...

### What should be done? https://www.mongodb.com/docs/manual/reference/operator/aggregation/firstN/#mongodb-group-grp.-firstN

### What should be done? https://www.mongodb.com/docs/manual/reference/operator/aggregation/max/ https://www.mongodb.com/docs/manual/reference/operator/aggregation/min/

### What should be done? https://www.mongodb.com/docs/manual/reference/operator/aggregation/last/#mongodb-group-grp.-last `$lastN` is moved to another issue https://github.com/FerretDB/FerretDB/issues/3197 ### Where? https://github.com/FerretDB/FerretDB/tree/main/internal/handlers/common/aggregations/operators/accumulators ### Definition of Done - all handlers updated; - integration/compatibility tests added/updated; - spot...

Implement `$first` accumulator

7

### What should be done? https://www.mongodb.com/docs/manual/reference/operator/aggregation/first/#mongodb-group-grp.-first `$firstN` was separated to issue https://github.com/FerretDB/FerretDB/issues/3196 DOD: - [ ] implement first accumulator - [ ] add integration tests for expression `{ $first: "$v"...

### What should be done? https://www.mongodb.com/docs/manual/reference/operator/aggregation/meta/#mongodb-expression-exp.-meta

### What should be done? https://www.mongodb.com/docs/manual/reference/operator/projection/slice/#-slice--projection-