Working with Databases and Collections

1. Creating Database

Action	Command	Notes
Switch / lazy create	`use mydb`	DB exists only after first write
Explicit (via insert)	`db.users.insertOne({})`	Creates DB + collection
From driver	`client.db("mydb")`	Lazy reference

Command	Output
`show dbs`	mongosh helper, lists with size
`db.adminCommand({listDatabases: 1})`	Full info
`db.getMongo().getDBNames()`	Names only
`listDatabases.filter`	Filter by criteria

Command	Effect
`use mydb; db.dropDatabase()`	Removes DB + all collections
`db.adminCommand({dropDatabase: 1})`	Same, explicit

Warning: Irreversible. Confirm with db.getName() first.

Approach	Command
Implicit	`db.users.insertOne({})`
Explicit	`db.createCollection("users")`
With validator	`db.createCollection("users", { validator: {...} })`
Capped	`db.createCollection("logs", { capped: true, size: 1048576 })`
Time series	`db.createCollection("m", { timeseries: { timeField: "ts" } })`

Command	Output
`show collections`	Names list
`db.getCollectionNames()`	Array of names
`db.getCollectionInfos()`	Full info: type, options, idIndex
`db.runCommand({listCollections: 1})`	Raw cursor

Command	Effect
`db.users.drop()`	Removes collection + indexes
`db.users.deleteMany({})`	Removes docs only, keeps indexes

Option	Description
`db.users.renameCollection("members")`	Same DB
`db.adminCommand({renameCollection: "a.x", to: "b.y"})`	Cross-DB (admin auth)
`dropTarget: true`	Replace existing target

db.users.stats({ scale: 1024 * 1024 });  // sizes in MB

db.createCollection("auditLog", { capped: true, size: 10485760, max: 5000 });

Option	Effect
`{validate: "users"}`	Basic structural check
`full: true`	Deep scan including indexes (slow)
`background: true`	Run without exclusive lock (7.0+)
`repair: true`	Attempt to fix corruption

Option	Purpose
validator	Schema validation expression
validationLevel	`strict` \| `moderate` \| `off`
validationAction	`error` \| `warn`
collation	Default collation
writeConcern	Default write concern
storageEngine	Per-engine options
expireAfterSeconds	TTL (time series only)