Techniques for referencing documents across collections

There are several techniques available to MongoDB developers. The more popular ones include the following:

• The building block approach
• Embedded documents
• Document references 

A less common cross-reference technique would be to use database references (DBRefs). For on-the-fly cross-references, another extremely useful tool is to use the $lookup pipeline aggregation operator. Although the building block approach has actually already been used in the scenarios describing Book Someplace, we present a short discussion of this approach next.