What does createdCollectionAutomatically mean in MongoDB?

The createdCollectionAutomatically field in MongoDB indicates whether an operation automatically created a collection that didn't exist. When you perform operations like createIndex() or insert() on a non-existing collection, MongoDB creates the collection automatically and returns this boolean flag as true.

Syntax

db.nonExistingCollection.createIndex({fieldName: 1});

// Returns:
{
    "createdCollectionAutomatically": true,
    "numIndexesBefore": 1,
    "numIndexesAfter": 2,
    "ok": 1
}

Example 1: Creating Index on Non-Existing Collection

Let us create an index on a collection that doesn't exist ?

db.createCollectionDemo.createIndex({"ClientCountryName": 1});

This will produce the following output ?

{
    "createdCollectionAutomatically": true,
    "numIndexesBefore": 1,
    "numIndexesAfter": 2,
    "ok": 1
}

Example 2: Adding Documents to the Collection

Now let us insert a document into the collection ?

db.createCollectionDemo.insertOne({
    "ClientName": "Larry",
    "ClientCountryName": "US"
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5cd2950be3526dbddbbfb612")
}

Verify the Collection

Display all documents from the collection ?

db.createCollectionDemo.find().pretty();

{
    "_id": ObjectId("5cd2950be3526dbddbbfb612"),
    "ClientName": "Larry",
    "ClientCountryName": "US"
}

Key Points

• createdCollectionAutomatically: true appears when MongoDB creates a collection during the operation.
• If the collection already exists, this field will be false or absent.
• Common operations that trigger automatic collection creation include createIndex(), insertOne(), and insertMany().

Conclusion

The createdCollectionAutomatically field helps you identify when MongoDB automatically created a collection during an operation. This is particularly useful for understanding whether your operation worked on an existing or newly created collection.