In this chapter, you learned how to conduct complex queries using the aggregation pipeline framework. You learned about stages, expression operators, and how to accumulate information such as sum, average, and so on. One of the most important aspects of the aggregation pipeline framework that you learned about in this chapter is the ability to access embedded objects or arrays.
You also learned about single-purpose aggregation (for example, sort and limit), as well as how to use map-reduce. You learned that, although map-reduce gives you flexibility in that JavaScript functions can be used, the aggregation framework is preferred as it uses native MongoDB methods and offers better performance.
In the next chapter, you will learn about how to maintain MongoDB performance.
