mongodb-usage（MongoDB 使用）

MongoDB Best Practices

MCP Limitation

This MCP operates in READ-ONLY mode. No write, update, or delete operations are possible.

Schema Design Patterns

Embedding vs Referencing

Embed when:

• Data is accessed together frequently
• Child documents are bounded (won't grow unbounded)
• One-to-few relationships
• Data doesn't change frequently

Reference when:

• Data is accessed independently
• Many-to-many relationships
• Documents would exceed 16MB limit
• Frequent updates to referenced data

Common Patterns

Subset pattern: Store frequently accessed subset in parent, full data in separate collection.

Bucket pattern: Group time-series data into buckets (e.g., hourly readings in one document).

Computed pattern: Store pre-computed values for expensive calculations.

Index Strategies

Index Guidelines

• Index fields used in queries, sorts, and aggregation $match stages
• Compound indexes support queries on prefix fields
• Covered queries (all fields in index) are fastest
• Too many indexes slow writes

Index Types

• Single field: Basic index on one field
• Compound: Multiple fields, order matters for queries
• Multikey: Automatically created for array fields
• Text: Full-text search on string content
• TTL: Auto-expire documents after time period

ESR Rule

For compound indexes, order fields by:

1. Equality (exact match fields)
2. Sort (sort order fields)
3. Range (range query fields like $gt, $lt)

Aggregation Pipeline

Performance Tips

• Put $match and $project early to reduce documents
• Use $limit early when possible
• Avoid $lookup on large collections without indexes
• Use $facet for multiple aggregations in one query

Common Stages

// Filter documents
{ $match: { status: "active" } }

// Reshape documents
{ $project: { name: 1, total: { $sum: "$items.price" } } }

// Group and aggregate
{ $group: { _id: "$category", count: { $sum: 1 } } }

// Sort results
{ $sort: { count: -1 } }

// Join collections
{ $lookup: { from: "orders", localField: "_id", foreignField: "userId", as: "orders" } }

Connection Best Practices

Connection String Formats

• Atlas: mongodb+srv://user:pass@cluster.mongodb.net/database
• Local: mongodb://localhost:27017/database
• Replica set: mongodb://host1,host2,host3/database?replicaSet=rs0

Connection Pooling

• Use connection pooling in applications (default in drivers)
• Set appropriate pool size for your workload
• Don't create new connections per request

Anti-Patterns to Avoid

• Unbounded arrays: Arrays that grow without limit
• Massive documents: Documents approaching 16MB
• Too many collections: Use embedding instead
• Missing indexes: Queries doing collection scans
• $where operator: Use aggregation instead for security
• Storing files in documents: Use GridFS for large files