Is it possible to mongodump the last "x" records from a collection?

Question

Can you use mongodump to dump the latest "x" documents from a collection? For example, in the mongo shell you can execute:

db.stats.find().sort({$natural:-1}).limit(10); 

Is this same capability available to mongodump?

I guess the workaround would be to dump the above documents into a new temporary collection and mongodump the entire temp collection, but would be great to just be able to do this via mongodump.

Thanks in advance,

Michael

Answer 1

mongodump does not fully expose the cursor interfaces. But you can work around it, using the --query parameter. First get the total number of documents of the collection

db.collection.count() 

Let's say there are 10000 documents and you want the last 1000. To do so get the id of first document you want to dump.

db.collection.find().sort({_id:1}).skip(10000 - 1000).limit(1) 

In this example the id was "50ad7bce1a3e927d690385ec". Now you can feed mongodump with this information, to dump all documents a with higher or equal id.

$ mongodump -d 'your_database' -c 'your_collection' -q '{_id: {$gte: ObjectId("50ad7bce1a3e927d690385ec")}}' 

UPDATE The new parameters --limit and --skip were added to mongoexport will be probably available in the next version of the tool: https://github.com/mongodb/mongo/pull/307

Answer 2

Building off of Mic92's answer, to get the most recent 1000 items from a collection:

Find the _id of the 1000th most recent item:

db.collection.find('', {'_id':1}).sort({_id:-1}).skip(1000).limit(1)

It will be something like 50ad7bce1a3e927d690385ec.

Then pass this _id in a query to mongodump:

$ mongodump -d 'your_database' -c 'your_collection' -q '{"_id": {"$gt": {"$oid": "50ad7bce1a3e927d690385ec"}}}'