mongodb move documents from one collection to another collection

Update 2

Please do NOT upvote this answer any more. As written @jasongarber's answer is better in any aspect.

Update

This answer by @jasongarber is a safer approach and should be used instead of mine.

Provided I got you right and you want to move all documents older than 1 month, and you use mongoDB 2.6, there is no reason not to use bulk operations, which are the most efficient way of doing multiple operations I am aware of:

> var bulkInsert = db.target.initializeUnorderedBulkOp()
> var bulkRemove = db.source.initializeUnorderedBulkOp()
> var date = new Date()
> date.setMonth(date.getMonth() -1)
> db.source.find({"yourDateField":{$lt: date}}).forEach(
    function(doc){
      bulkInsert.insert(doc);
      bulkRemove.find({_id:doc._id}).removeOne();
    }
  )
> bulkInsert.execute()
> bulkRemove.execute()

This should be pretty fast and it has the advantage that in case something goes wrong during the bulk insert, the original data still exists.

Edit

In order to prevent too much memory to be utilized, you can execute the bulk operation on every x docs processed:

> var bulkInsert = db.target.initializeUnorderedBulkOp()
> var bulkRemove = db.source.initializeUnorderedBulkOp()
> var x = 10000
> var counter = 0
> var date = new Date()
> date.setMonth(date.getMonth() -1)
> db.source.find({"yourDateField":{$lt: date}}).forEach(
    function(doc){
      bulkInsert.insert(doc);
      bulkRemove.find({_id:doc._id}).removeOne();
      counter ++
      if( counter % x == 0){
        bulkInsert.execute()
        bulkRemove.execute()
        bulkInsert = db.target.initializeUnorderedBulkOp()
        bulkRemove = db.source.initializeUnorderedBulkOp()
      }
    }
  )
> bulkInsert.execute()
> bulkRemove.execute()

The bulk operations @markus-w-mahlberg showed (and @mark-mullin refined) are efficient but unsafe as written. If the bulkInsert fails, the bulkRemove will still continue. To make sure you don't lose any records when moving, use this instead:

function insertBatch(collection, documents) {
  var bulkInsert = collection.initializeUnorderedBulkOp();
  var insertedIds = [];
  var id;
  documents.forEach(function(doc) {
    id = doc._id;
    // Insert without raising an error for duplicates
    bulkInsert.find({_id: id}).upsert().replaceOne(doc);
    insertedIds.push(id);
  });
  bulkInsert.execute();
  return insertedIds;
}

function deleteBatch(collection, documents) {
  var bulkRemove = collection.initializeUnorderedBulkOp();
  documents.forEach(function(doc) {
    bulkRemove.find({_id: doc._id}).removeOne();
  });
  bulkRemove.execute();
}

function moveDocuments(sourceCollection, targetCollection, filter, batchSize) {
  print("Moving " + sourceCollection.find(filter).count() + " documents from " + sourceCollection + " to " + targetCollection);
  var count;
  while ((count = sourceCollection.find(filter).count()) > 0) {
    print(count + " documents remaining");
    sourceDocs = sourceCollection.find(filter).limit(batchSize);
    idsOfCopiedDocs = insertBatch(targetCollection, sourceDocs);

    targetDocs = targetCollection.find({_id: {$in: idsOfCopiedDocs}});
    deleteBatch(sourceCollection, targetDocs);
  }
  print("Done!")
}