Creating indexes in NoSQL / Document-based databases is quite different compared to traditional relational databases. Since the former are schema-less (there is no table creation), indexes do not get created when the collection or the document is created or inserted. Here is a quick tip that will create all the indexes defined in your documents when using Symfony 2 and Doctrine ODM for MongoDB. Indexes are a great way to speed up your queries, in fact, it is a crime not to include them in your documents.
Adding the index is as easy as putting a new annotation on the property in its document class (for @Index) and run a bit of code in Symfony to build it out. The two lines you'll need to execute are included in the post.