mongodb - random document(s)

gistfile1.js

// mongodb collection generator for getting random documents...

    // set to the collection you wish to add a random attribute to.
var collection      = 'myCollection', 
    // the name of the random collection to be generated; what you'll run queries against
    // to obtain a random document
    randCollection  = 'rand' + collection.charAt(0).toUppderCase(),
    // wheter or not the random collection should contain full documents, or a truncated
    // version of the document (the mongo $_id and random attribute only)
    useTruncatedDoc = false;

db[collection].find().forEach(function(doc){
    
    if (useTruncatedDoc) {
        doc = {
            _id: doc._id
        };
    }

    doc.randomAttr = Math.random(),
    db[randCollection].insert(doc); 
});

// generate an index on the new collection..
db[randCollection].ensureIndex({
    randomAttr: 1
});


/**
 // To grab a single random document after running the above code:
 db[randCollection].findOne({
    randomAttr: {
      $gte: Math.random()
    }
  });
**/