Aggregation simply means collection of related items of content which are brought together so that they can be displayed as per the requirements. It literally means processing data records in a logical manner to return the desired output.
It works on the concept of data processing through pipelines. The documents enter the multi-stage pipeline that transforms the data into aggregated results.
Stages of Pipeline
First Stage – The $match stage filters the documents by the field name and passes data to the next stage.
Second Stage – The $group stage groups the documents by the field name to show the desired output.
The aggregate() method in mongoDB
We use the aggregate method in mongoDB as mentioned below :
Pipeline operators in mongoDB :
1) $project – This function passes only the specified fields to the next stages of the pipeline.
2) $match – This function filters the data according to the specified condition . It passes only the filtered data to next stages of pipeline.
3) $unwind – It is used to output a document for each element of an array.
4) $group – Aggregation operations group values from multiple documents together, and can perform a variety of operations on the grouped data to return a single result.
Aggregation pipeline can be used as illustrated below: