Powerful framework for running multiple operations in a pipeline, mostly replacing M/R:
- • $match: for matching documents, à la find().
- • $project: for "rewriting" documents, but more powerful with
computed expressions: adding fields ($add),
string functions ($toLower), etc
- • $unwind: hands out array elements each at a time
- • $group: aggregates items into buckets defined by key
- • $sort, $limit, and $skip
