ruby on rails - Implement autocomplete on MongoDB

Question

Say I have a collection of users and want to implement autocomplete on the usernames of those users. I looked at the mongodb docs and $regex seems to be one way to do this. Is there a better way? By better I mean more performant/better practice.

Answer 1

As suggested by @Thilo, you can use several ideas including prefixing.

The most important thing is to have very quick request (because you want autocomplete to feel instaneous). So you have to use query which will use properly indexes.

With regexp : use /^prefix/ (the important thing is the ^ to specify the beginning of line which is mandatory to make the query use index).

The range query is good too : { $gt : 'jhc', $lt: 'jhd' } }

More complicated but faster : you can store prefix-trees in mongo (aka tries) with entries like :

 {usrPrefix : "anna", compl : ["annaconda", "annabelle", "annather"]}
 {usrPrefix : "ann", compl : ["anne", "annaconda", "annabelle", "annather"]}

This last solution is very fast (if indexes on compl of course) but not space efficient at all. You know the trade-off you have too choose.