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