How to get all the collections from all the MongoDB databases?

To get all the collections from all MongoDB databases, you need to first retrieve all databases and then iterate through each database to get their collections.

Syntax

// Step 1: Get all databases
switchDatabaseAdmin = db.getSiblingDB("admin");
allDatabaseName = switchDatabaseAdmin.runCommand({ "listDatabases": 1 }).databases;

// Step 2: Iterate through databases to get collections
allDatabaseName.forEach(function(databaseName) {
    db = db.getSiblingDB(databaseName.name);
    collectionName = db.getCollectionNames();
    // Process collections as needed
});

Method 1: Get All Database Names

First, let us get all the databases using the admin database ?

switchDatabaseAdmin = db.getSiblingDB("admin");
allDatabaseName = switchDatabaseAdmin.runCommand({ "listDatabases": 1 }).databases;

This will produce the following output ?

[
    {
        "name" : "admin",
        "sizeOnDisk" : 495616,
        "empty" : false
    },
    {
        "name" : "config", 
        "sizeOnDisk" : 98304,
        "empty" : false
    },
    {
        "name" : "local",
        "sizeOnDisk" : 73728,
        "empty" : false
    },
    {
        "name" : "sample",
        "sizeOnDisk" : 1335296,
        "empty" : false
    },
    {
        "name" : "sampleDemo",
        "sizeOnDisk" : 278528,
        "empty" : false
    },
    {
        "name" : "studentSearch",
        "sizeOnDisk" : 262144,
        "empty" : false
    },
    {
        "name" : "test",
        "sizeOnDisk" : 8724480,
        "empty" : false
    }
]

Method 2: Get All Collections from All Databases

Following query iterates through each database and retrieves all collection names ?

allDatabaseName.forEach(function(databaseName) {
    db = db.getSiblingDB(databaseName.name);
    collectionName = db.getCollectionNames();
    collectionName.forEach(function(collectionName) {
        print(collectionName);
    });
});

This will produce the following output with all collection names ?

clearingItemsInNestedArrayDemo
customIdDemo
deleteRecordDemo
documentExistsOrNotDemo
findAllExceptFromOneOrtwoDemo
mongoExportDemo
startup_log
arraySizeErrorDemo
basicInformationDemo
copyThisCollectionToSampleDatabaseDemo
deleteAllRecordsDemo
deleteDocuments
deleteDocumentsDemo
employee
findListOfIdsDemo
userInformation
updateInformation
sourceCollection
prettyDemo

Key Points

• getSiblingDB("admin") switches to the admin database which has privileges to list all databases
• runCommand({ "listDatabases": 1 }) retrieves metadata for all databases in the MongoDB instance
• getCollectionNames() returns an array of collection names for the current database
• The forEach loop iterates through each database and extracts its collections

Conclusion

Use the admin database to list all databases, then iterate through each database using getSiblingDB() and getCollectionNames() to retrieve all collections across your MongoDB instance.