In this post, we will learn how to use javascript as MongoDB shell script to export (fields of) a collection.

If you just want to execute MongoDB commands then look at the another post there..

OK, so let’s focus back to the objective of this post.
Generally, to export data from a MongoDB collection, we use the following command -

mongoexport -vvvv --host 127.0.0.1 --db my-db --username='usr' --password='pwd' --collection profiles --csv --out /home/data/usernames.csv -f 'username'

Definitely I prefer the above mentioned way. But let’s consider that you have millions or billions of records in a MongoDB collection. And you want to export all these.
Probably, this will just hang. Well, I am not sure what happens on your computer but on my machine it was just stuck for hours and then I terminated the process. And came up with scripting solution – where I created multiple files (chunks) of the exported data.

var size = 1000000;
var maxCount = 1;
for (x=0;x<maxCount;x=x+1)
{
   var recToSkip = x*size;
   db.profiles.find().skip(recToSkip).limit(size).forEach( function(record){
     var username = record.username;
     print(record.username);
   })
}

And ofcourse to run the script (if above javascript is in file named – ‘usernames.js’)

  mongo 127.0.0.1/my-db --username='username' --password='pwd' export-usernames.js > output.txt

The above shell command will render all the output (from export-usernames.js) to the output.txt file. And mind it, that this is single file where you get all exported data. But if you want to chunkify this – just write your wrapper script or run multiple times. It’s upto you.

I hope this might help someone. Or please feel free to add your comments.

Cheers!
Salil Kalia
Salil [at] IntelliGrape [dot] com
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