From compose.io’s blog
The core tool in MongoDB’s analytics story is aggregation – it lets your create a pipeline through which your selected documents can be passed which can then perform various operations to burn down to the information you are after. You could say aggregate your orders table, and in the pipeline, have an operator which matched only a particular classes of product ordered, then another operator that grouped the total sales of each product class. The pipeline has until now only worked with one collection’s documents… it’s into this pipeline that MongoDB 3.2 is adding a $lookup operator to bring in data from another collection.
