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LIMITATIONS.md

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Known Limitations

Not implemented (raise NotImplementedError)

  • Change streams (watch())
  • find_raw_batches(), aggregate_raw_batches()
  • Sessions and transactions (basic ClientSession exists but limited)
  • $out with aggregation (basic support only)
  • Full $merge stage support
  • Collation on most operations
  • hint parameter on delete operations
  • $query/$orderby legacy format

Behavioral differences

  • aggregate() returns list instead of CommandCursor
  • TTL indexes are simulated via simulate_ttl() — not automatic
  • No network overhead — all in-memory
  • SERVER_VERSION defaults to 7.0.34 (configurable via env MONGODB)
  • ObjectId generation is not guaranteed globally unique (mock)
  • Write concerns are accepted but always acknowledged

Partial implementations

  • $lookup pipeline syntax works but limited sub-pipeline support
  • $setWindowFields — basic window functions, not all boundaries
  • $fill — linear/method not implemented
  • $graphLookup — depth-first only, no maxDepth validation
  • Geospatial: 2dsphere index simulated, only GeoJSON geometries
  • $convert — most conversions work, some edge cases differ

Not planned

  • Real network/replica set behavior
  • Sharding
  • Actual persistent storage