Handy MongoDB Utility Methods

Find by _id / ObjectId

The _id field is stored as a special ObjectId type, not a plain string — you must wrap the value in ObjectId(...) for the query to match.

db.employees.find({ _id: "64ba88c4768ace4777482ea1" })
// returns nothing — _id is not a plain string

db.employees.find({ _id: ObjectId("64ba88c4768ace4777482ea1") })
// this is the correct way to query by _id

validate()

Scans a collection's data and indexes for corruption and returns a detailed report.

db.employees.validate()
// {
//   ns: 'rahultechnology.employees',
//   nInvalidDocuments: 0,
//   nrecords: 9,
//   valid: true,
//   ...
// }

db.employees.validate({ full: true })   // deeper, more thorough scan
db.employees.validate({ repair: true }) // attempt to repair issues found

Checking Collection Storage

Three quick methods tell you how much disk space a collection is using:

db.employees.storageSize()
// total bytes allocated for storing documents

db.employees.totalSize()
// document data size + all index sizes combined

db.employees.totalIndexSize()
// total size of all indexes on the collection

Common Errors & Troubleshooting

• find({ _id: "some-hex-string" }) returning nothing — this is the single most common MongoDB beginner mistake — _id is an ObjectId, not a string; wrap it: find({ _id: ObjectId("some-hex-string") }).
• ObjectId throwing an error on an invalid string — ObjectId() requires exactly a 24-character hex string; passing anything else (like a UUID, or a string that's the wrong length) throws immediately.
• validate() reporting valid: false — this indicates real corruption; back up the collection immediately and consider { repair: true }, or restoring from a recent backup if repair doesn't resolve it.

Key Takeaways

• Always wrap _id lookups in ObjectId(...) — a plain string will never match.
• validate() is your go-to health check when a collection behaves unexpectedly.
• storageSize(), totalSize(), and totalIndexSize() help you keep an eye on disk usage per collection.