mongodb move documents from one collection to another collection

Question

How can documents be moved from one collection to another collection in MongoDB?? For example: I have lot of documents in

Answer 1

I had 2297 collection for 15 million of documents but some collection was empty.

Using only copyTo the script failed, but with this script optimization:

db.getCollectionNames().forEach(function(collname) {
    var c = db.getCollection(collname).count();
    if(c!==0){
      db.getCollection(collname).copyTo('master-collection');
      print('Copied collection ' + collname);
    }
});

all works fine for me.

NB: copyTo is deprecated because it block the read/write operation: so I think is fine if you know that the database is not usable during this operation.