Now even if there are some basic ways (see  and ) to achieve this with MongoDB alone, people are still looking for more scalable solutions as shown by this thread ☞ covering Solr integration with MongoDB. The thread also mentions a couple of existing Ruby or Rails plugins for this integration.
One concern that I’ve expressed about the integration with Lucene alone is that you’ll have to deal with its scalability. Solr is one way to do that automatically. Lately I have heard of a new solution for scalable search: ☞ ElasticSearch which sounds quite interesting (nb: I haven’t yet gone through its docs or played with it, but the creator of the project has a long search/indexing history behind. You can find more details about Elastic Search here).