MongoDB Error 48

Error code

48

Code name

NamespaceExists

Product

MongoDB

Versions

MongoDB 7.0 and 8.0 (wordings checked against the 8.0 source)

Category

Indexes & Collections

Last reviewed

24 October 2026

What it means

Error 48 (NamespaceExists) means a create or rename targets a database.collection name that is already taken, by a collection with different options, a view or a time series collection, so MongoDB refused to replace it.

In MongoDB 8.0 creating a collection that already exists with the same options is not an error: outside a transaction, the create command compares the requested options with the existing collection's and returns success when they match. You get 48 when they do not match, and the message shows the existing collection's options after but with different options:, so you can see what differs (for example a validator, capped settings or a collation). Inside a multi-document transaction the server returns the plain Collection ... already exists. form instead, because the error aborts the transaction.

A name can also be taken by something that is not an ordinary collection: a view, or a time series collection, which the messages name explicitly. For renames, renameCollection refuses to overwrite an existing target unless you pass dropTarget: true, and the documentation shows the operation failing when the target name is an existing collection.

The usual setting is a migration or start-up script that runs on every deploy. If it creates collections with options (validators, time series, collation), the second run fails as soon as anyone has changed those options, by hand or through collMod.

Common causes

• A start-up or migration script calls createCollection with options that differ from those of the existing collection.
• The collection was created implicitly (by an insert or createIndex) with default options, and a later explicit createCollection adds a validator or other options.
• A view or time series collection already uses the name.
• renameCollection targets a name that is already in use, without dropTarget.
• createCollection runs inside a transaction for a collection that already exists.

Diagnose it

Read-only These mongosh commands only read data, indexes, statistics and server state (find, aggregate without $out or $merge, getIndexes, stats, currentOp, rs.status). They do not change data, users or settings, so they are safe to run first.

db.getCollectionInfos({ name: "events" })

db.getCollectionNames().filter(n => n === "events_old")

db.events_old.estimatedDocumentCount()

How to fix it

Each fix is labelled with what it changes. Try read-only checks and the least invasive fix first, and test changes outside production.

db.runCommand({
  collMod: "events",
  validator: { $jsonSchema: { bsonType: "object", required: [ "type", "at" ] } },
  validationLevel: "moderate"
})

if (!db.getCollectionNames().includes("events")) {
  db.createCollection("events", { validator: { $jsonSchema: { bsonType: "object" } } })
}

db.events.renameCollection("events_archive_2026_10")

db.events_new.renameCollection("events", true)

Examples

Adding a validator by re-creating the collection

Not reproduced here: no MongoDB server is installed for this site. The text is built by checkCollectionOptions() in MongoDB 8.0's create_command.cpp; the part after "different options:" is the existing collection's options document, shortened here. Running the same createCollection with identical options would succeed.

db.events.insertOne({ type: "click" })      // creates events with default options

db.createCollection("events", {
  validator: { $jsonSchema: { bsonType: "object", required: [ "type" ] } }
})

MongoServerError: namespace shop.events already exists, but with different options: { ... }

Renaming onto an existing collection

Not reproduced here: no MongoDB server is installed for this site. The text is the fixed string in MongoDB 8.0's rename_collection.cpp for a rename within one database on a replica set or standalone server; on a sharded cluster the check runs in the sharding rename code (sharding_ddl_util.cpp) and can use the dropTarget wording shown in the variants.

db.events_new.renameCollection("events")

MongoServerError: target namespace exists

How to prevent it

Create collections that need options in one migration step that runs once, and change options later with collMod, never by calling createCollection again with new options.

In application start-up code, either skip explicit creation or check getCollectionInfos() first, and treat code 48 as a signal to compare options rather than as something to ignore.

Common questions

Official documentation

• MongoDB error codes (48 NamespaceExists)
• db.createCollection()
• db.collection.renameCollection()
• collMod

Last technically reviewed 24 October 2026; examples and diagnostics run on MongoDB 8.0 documentation and source (error_codes.yml r8.0.0); not run locally. Codes and code names are checked against MongoDB's error_codes.yml (8.0); no MongoDB server was available, so the examples were not run locally.