database-mongodb

name	database-mongodb
description	Apply expert schema design, indexing, and performance rules for MongoDB. Use when designing MongoDB schemas, creating indexes, or optimizing NoSQL query performance.
metadata	{"triggers":{"files":["**/*.ts","**/*.js","**/*.json"],"keywords":["mongo","mongoose","objectid","schema","model"]}}

MongoDB Best Practices

Priority: P0 (CRITICAL)

Schema Design

• Embed vs Reference:
• Embed (1:Few): Addresses, Phone Numbers. Optimization: Read locality.
• Reference (1:Many/Infinity): Logs, Activity History. Optimization: Document size limits (16MB).
• Bucket Pattern: For time-series or high-cardinality "One-to-Many", bucket items into documents (e.g., DailyLog).

Optimize Indexes

• ESR Rule: Equality, Sort, Range. Order your index keys (status, date, price) if you query status='A', sort by date, filter price > 10.

See implementation examples for compound index and pagination patterns.

• Text Search: Use $text search instead of $regex for keywords. $regex slow (linear scan) unless anchored (^prefix).
• Covered Queries: Project only indexed fields to avoid fetching document (PROJECTION key).
• Explain Plan: Target nReturned / keysExamined ratio of ~1. If docsExamined >> nReturned, index inefficient.

Scale with Sharding

• Shard Key: Avoid monotonically increasing keys (e.g., Timestamp, ObjectId) for high-write workloads (creates "Hot Shards"). Use Hashed Sharding or high-cardinality natural keys.

Improve Query Performance

• Cursor-Based Pagination: Use _id or sort-key based pagination instead of skip(). skip(10000) scans 10000 docs.

• Aggregation: Prefer Aggregation Framework ($match, $group) over bringing data to client (JS).

Configure Operations

• Write Concern: Understand w:1 (Ack) vs w:majority (Safe).
• Transactions: Use only when ACID across multiple documents stricter than performance needs.

Anti-Patterns

• No unbounded arrays: Use $push with $slice or redesign using Bucket Pattern.
• No client-side filtering: Project only needed fields; never fetch full docs to filter in memory.
• No deep nesting: Keep nesting ≤4 levels; flatten paths that frequently queried.

References

• Best Practices Guide
• Anti-Patterns
• Postgres vs Mongo Comparison