mongodb - Ordering a result set randomly in mongo

Question

I've recently discovered that Mongo has no SQL equivalent to "ORDER BY RAND()" in it's command syntax (https://jira.mongodb.org/browse/SERVER-533)

I've seen the recommendation at http://cookbook.mongodb.org/patterns/random-attribute/ and frankly, adding a random attribute to a document feels like a hack. This won't work because this places an implicit limit to any given query I want to randomize.

The other widely given suggestion is to choose a random index to offset from. Because of the order that my documents were inserted in, that will result in one of the string fields being alphabetized, which won't feel very random to a user of my site.

I have a couple ideas on how I could solve this via code, but I feel like I'm missing a more obvious and native solution. Does anyone have a thought or idea on how to solve this more elegantly?

Answer 1

I have to agree: the easiest thing to do is to install a random value into your documents. There need not be a tremendously large range of values, either -- the number you choose depends on the expected result size for your queries (1,000 - 1,000,000 distinct integers ought to be enough for most cases).

When you run your query, don't worry about the random field -- instead, index it and use it to sort. Since there is no correspondence between the random number and the document, you should get fairly random results. Note that collisions will likely result in documents being returned in natural order.

While this is certainly a hack, you have a very easy escape route: given MongoDB's schema-free nature, you can simply stop including the random field once there is support for random sort in the server. If size is an issue, you could run a batch job to remove the field from existing documents. There shouldn't be a significant change in your client code if you design it carefully.

An alternative option would be to think long and hard about the number of results that will be randomized and returned for a given query. It may not be overly expensive to simply do shuffling in client code (i.e., if you only consider the most recent 10,000 posts).