On his blog today Jonathan Wage has posted a tip on getting MongoDB connections and queries to work through the Doctrine ORM layer:

Since the start of the Doctrine MongoDB Object Document Mapper project people have asked how it can be integrated with the ORM. This blog post demonstrates how you can integrate the two transparently, maintaining a clean domain model. This example will have a Product that is stored in MongoDB and the Order stored in a MySQL database.

His code shows how to define the document and entity for the connection (a Product and Order) and creating an event subscriber to lazy load the product. He creates a sample Product and an Order for it and save them to the database. He also includes code to pull an order back out by its ID number and get an Order object back out (with Product data inside).