mongodb command line function to add a 'rand' field to every document in a collection.

gistfile1.js

function add_rand_field(collection) {
    collection.find().forEach(function(data) {
        collection.update({_id:data._id}, {
          $set:{rand:Math.random()}
        });
    });
    collection.ensureIndex({rand:1},{background:true})
}

add_rand_field(db.mycoll);