Mastering MongoDB 4.x - Second Edition

Aggregation stage operators

An aggregation pipeline is composed of different stages. These stages are declared in an array and executed sequentially, with the output of every stage being the input of the next one.

The $out stage has to be the final stage in an aggregation pipeline, outputting data to an output collection by replacing or adding to the existing documents:

• $group: This is most commonly used to group by identifier expression and to apply the accumulator expression. It outputs one document per distinct group.
• $project: This is used for document transformation, and outputs one document per input document.
• $match: This is used for filtering documents from input based on criteria.
• $lookup: This is used for filtering documents from ...