MongoDB Aggregation Framework


Lynn, John (2013-06-17). MongoDB Aggregation Framework Principles and Examples

What is the aggregation framework?

The aggregation framework was created to provide a “means to calculate aggregated values without having to use map-reduce” according to the introduction in the MongoDB documentation at: http:// manual/ core/aggregation/# overview . Not that there’s anything wrong with using map-reduce, but it’s often viewed as a complicated way to do a simple thing – calculate aggregate values. The aggregation framework simply offers a easier way to do this simple thing.”

A simple example: US states with population over 10 million

{$ group:{ _id:” $ state”, totalPop:{ $ sum:” $ pop”}}},
{$ match:{ totalPop:{ $ gte: 10* 1000* 1000}}}

The idea here is that the pipeline operators flow results from one operator to the next . So in this example, the documents from the zipcodes collection in its entirety are presented to the $group operator to work on; And then the documents coming out of $group are presented as input to $match.

// skip 1 document, starting from the beginning of the set of documents, and then output the rest. This will output all but the first document, which may be a lot of output
printjson( db.zipcodes.aggregate( { $ skip : 1} ))

In our northwind sample collection of customer orders, notice that the line items in each order are contained in an array called orderItems. Each item in this array is a sub-document with the product category, supplier, product name, and pricing information contained in unitPrice, as well as quantity of the item ordered. A common aggregation to be performed is: calculating order total – the total for each order, based on the line items.
{$ match: {” orderId”: 10253}}
, {$ unwind: “$ orderItems”}

Now that the document for order 10253 has been exploded into 3 documents by $ unwind, a simple $ group operator can be utilized to get an order total.

db.northwind.aggregate( {$ match: {” orderId”: 10253}} , {$ unwind: “$ orderItems”} , {$ group: {_id: “$ orderId”, “OrderTotal”: {$ sum: “$ orderItems.unitPrice”

$ project
The $ project operator is used to “re-shape” the documents, inserting and deleting fields or whole sub-documents; creating computed values using existing fields or constants as input; conditionally including parts of documents, and more. Using $ project, you can do some very creative manipulation of the documents in the pipeline.



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