mongodb

name	mongodb
description	MongoDB — schema design, indexes, aggregation, sharding, Atlas.

MongoDB Reference

When to Load

Load when the project uses MongoDB (signal: mongoose, mongodb, mongo: in docker-compose, MONGODB_URI).

CLI Connection

mongosh "$MONGO_URI"
# or with individual vars:
mongosh "mongodb://$DB_USER:$DB_PASSWORD@$DB_HOST:${DB_PORT:-27017}/$DB_NAME"
# Atlas:
mongosh "mongodb+srv://$DB_USER:$DB_PASSWORD@$ATLAS_CLUSTER/$DB_NAME"

Key inspection commands:

db.stats()                                  // database size and counts
db.collection.stats()                       // collection storage details
db.collection.explain("executionStats").find({...})  // query plan
db.currentOp({ active: true })              // running operations
db.collection.getIndexes()                  // list all indexes

Schema Design Principles

Decision	Embed	Reference
Data is always accessed together	yes	—
Data grows unboundedly (comments on a post)	—	yes
Data is shared across many documents	—	yes
Max document size	—	16 MB limit; embed only bounded sub-arrays

Rule of thumb: embed for one-to-few; reference for one-to-many or many-to-many.

Indexes

Type	Use when
Single-field	Filter or sort on one field
Compound	Multi-field queries; field order matches query selectivity
Multikey	Index array values; MongoDB creates one entry per array element
Text	Full-text search ($text); use Atlas Search for production
2dsphere	Geospatial queries ($near, $geoWithin)
Partial	partialFilterExpression to index a subset of documents
Sparse	Skip documents where the field is absent
TTL	Auto-delete documents after a duration: expireAfterSeconds

Always run explain("executionStats") to verify IXSCAN (not COLLSCAN).

Aggregation Pipeline

Prefer aggregation over MapReduce (deprecated in 5.0+):

• $match → $group → $sort → $limit (always filter early, sort late)
• $lookup for joins (use only for occasional aggregations; not for hot paths)
• $unwind before $group when working with arrays
• allowDiskUse: true for large aggregations that exceed 100 MB memory limit

Write Concerns & Read Preferences

Concern	Guarantee
w: 1 (default)	Acknowledged by primary
w: "majority"	Acknowledged by majority of replica set members
j: true	Written to journal before acknowledgement

For financial or critical data: use w: "majority", j: true.

Read preferences: primary (default, strong consistency), primaryPreferred, secondary (eventual consistency, lower load on primary).

Sharding

• Shard key choice is critical and cannot be changed after collection creation
• Good shard key: high cardinality, even distribution, frequently in queries
• Bad shard key: low cardinality (e.g., boolean), monotonically increasing (e.g., timestamp → hotspot)
• Use hashed shard key for even distribution when range queries are rare

Transactions

Available since MongoDB 4.0 (replica sets) and 4.2 (sharded clusters):

• Use sparingly; multi-document transactions have performance overhead
• Keep transactions short; avoid user interactions inside a transaction
• Prefer single-document atomic operations where possible

Atlas-Specific

Feature	Use when
Atlas Search	Full-text, fuzzy, autocomplete (Lucene-backed)
Atlas Triggers	React to DB events without polling
Atlas Data API	HTTP access without MongoDB driver
Online Archive	Auto-tier cold data to S3

Common Gotchas

Gotcha	Fix
findOne returns null not an exception	Always check for null before accessing fields
_id is an ObjectId, not a string	Serialize with .toString(); never compare == to a string
Unbounded array growth	Use $push with $slice or move to a separate collection
Schema-less does not mean schema-free	Use Mongoose validators or JSON Schema validation
Large $in arrays cause slow index scans	Cap $in lists; consider $lookup + $match