Retrieve unique random items from a mongodb collection? - mongodb

Question

I run an IRC bot and I have a function which returns 1 random url using Math.random at the moment, from my Mongodb collection.
I would like to refactor it to return x number of unique items, and for each subsequent invocation of the url fetching command .getlinks I would like that it keeps everything unique, so that a user doesn't see the same link unless all the possible links have been already returned.
Is there some algorithm or native mongodb function I could use for this?
Here's a sample scenario:
I have a total of 9 records in the collection. They have a _id and url field.
user a: .getlinks()
bot returns: http://unique-link-1, http://unique-link-2, http://unique-link-3, http://unique-link-4
user a: .getlinks()
bot returns: http://unique-link-5, http://unique-link-6, http://unique-link-7, http://unique-link-8
user a: .getlinks()
bot returns: http://unique-link-9, http://unique-link-6, http://unique-link-1, http://unique-link-3
Background information:
There's a total of about 200 links. I estimate that will grow to around 5000 links by the end of next year.
Currently the only thing I can think of is keeping an array of all returned items, and grabbing all items from the collection at once and getting a random one 4 times and making sure it's unique and hasn't been shown already.
var shown = [], amountToReturn = 4;
function getLinks() {
var items = links.find(), returned = [];
for ( var i = 0; i<amountToReturn; i++ ) {
var rand = randItem( items );
if ( shown.indexOf( rand.url ) == -1 && shown.length < items.length ) ) {
returned.push( rand.url );
}
}
message.say( returned.join(',') );
}

Answer 1

You should find a number of possible options to get random item(s) from Collection here ...
http://jira.mongodb.org/browse/SERVER-533
Another intersting method is documented here ...
http://cookbook.mongodb.org/patterns/random-attribute/
The method mentioned above basically creates a new key/value on the document using Math.random()
> db.docs.drop()
> db.docs.save( { key : 1, ..., random : Math.random() } )
> db.docs.save( { key : 1, ..., random : Math.random() } )
> db.docs.save( { key : 2, ..., random : Math.random() } )
... many more insertions with 'key : 2' ...
> db.docs.save( { key : 2, ..., random : Math.random() } )
...

Answer 2

Get random records form mongodb via map/reduce
// map
function() {
emit(0, {k: this, v: Math.random()})
}
// reduce
function(k, v) {
var a = []
v.forEach(function(x) {
a = a.concat(x.a ? x.a : x)
})
return {a:a.sort(function(a, b) {
return a.v - b.v;
}).slice(0, 3 /*how many records you want*/)};
}
// finalize
function(k, v) {
return v.a.map(function(x) {
return x.k
})
}