Advanced use of expressions for array processing
One of the most compelling aspects of MongoDB is the ability to embed arrays within documents. Unlike relational databases, this characteristic allows each entity's entire data structure to exist in one place as a document.
The aggregation framework provides a rich set of aggregation operator expressions for analyzing and manipulating arrays. When optimizing for performance, array expressions are critical because they prevent the unwinding and regrouping of documents when you only need to process each document's array in isolation. For most situations when you need to manipulate an array, there is usually a single array operator expression that you can utilize for your requirements.
Occasionally, you won't be able to use a single out-of-the-box array operator expression to solve an array processing challenge. Consequently, you must assemble a composite of nested lower-level expressions to handle the challenging array...
