Copying a mongo collection using Java Driver

Question

I want to copy the contents from one collection to another.

in mongod this can be done:

db.tempMongoItem.find().forEach( function(x){db.mongoItem.insert(x)} )

Using Java Mongo Driver, I try:

DB db = mongoClient.getDB("mydb")

CommandResult result = db.command("db.tempMongoItem.find().forEach( function(x){db.mongoItem.insert(x)} )")

But I get:

result = [serverUsed:localhost:27017, ok:0.0, errmsg:no such cmd: db.tempMongoItem.find().forEach( function(x){db.mongoItem.insert(x)} ), code:59, bad cmd:[db.tempMongoItem.find().forEach( function(x){db.mongoItem.insert(x)} ):true]]

Any ideas?

Answer 1

You need to emulate the same thing JS is doing in Java, which means getting a cursor and iterating over it, inserting each document into new collection.

Something like this (coll is current, coll2 is new collection):

DBCursor cursor = coll.find();
try {
   while(cursor.hasNext()) {
       coll2.insert(cursor.next());
   }
} finally {
   cursor.close();
}

Both coll and coll2 are assumed to be DBCollection type.

Since it appears you are copying within the same DB, there is another way to do this if you are using 2.6 MongoDB using aggregation framework $out stage:

db.collection.aggregate({"$out":"newCollection"});

Note that this is limited to outputting into the same DB that original collection is in.