As we have discussed throughout this book, one major objective in your initial database design should be to design a database document structure such that there is no need to cross-reference data between collections. The temptation, especially for those of us steeped in traditional RDBMS database techniques, is to normalize the database structure and then define a series of relationships that are contingent upon a series of interlocking foreign keys. This is the beginning of many problems ... for legacy RDBMS developers! For those of us fortunate enough to be operating in the MongoDB NoSQL world, such nightmares are a thing of the past, given a proper design. Even with a proper database design, however, you might still find occasions requiring a reference to documents across collections in MongoDB.

Let's first have a look at a number of techniques commonly used to reference documents across collections.