mongodb select from different databases

In Mongo shell, this can be done by using db.getSiblingDB() method to switch to admin database and get a list of the 200 databases by running the admin command db.runCommand({ "listDatabases": 1 }). Iterate over the list of databases and use db.getSiblingDB() again to switch between databases, query the Group collection for the meldingId values. Something like this:

// Switch to admin database and get list of databases.
db = db.getSiblingDB("admin");
dbs = db.runCommand({ "listDatabases": 1 }).databases;

// Iterate through each database.
dbs.forEach(function(database) {
    db = db.getSiblingDB(database.name);

    // Get the Group collection
    collection = db.getCollection("Group");

    // Iterate through all documents in collection.
    /*
        collection.find().forEach(function(doc) {

            // Print the meldingId field.
            print(doc.meldingId);
        });
    */

    var meldingIds = collection.distinct('meldingId');
    print(meldingIds);

});

MongoDB query multiple databases at once The same as @chridam answer, but with small improvement to generalize the db name, and as a one-liner: This command will count all the collection with the same name in different databases:

collectionName = "sites"; db = db.getSiblingDB("admin"); dbs = db.runCommand({ "listDatabases": 1 }).databases; dbs.forEach(function(database) { db = db.getSiblingDB(database.name); collection = db.getCollection(collectionName); print(collection + " count: " + collection.count())})

And the results are:

db_0.sites count: 0
db_1.sites count: 0
db_2.sites count: 3
db_3.sites count: 4

You can use it for different purposes just change the collectionName value, or the query in the forEach function.