MongoDB Aggregation Framework

• Aggregation framework is just another way of querying data in mongodb.

• It enables you to group data, perform calculations, reshape data etc.

• It works based on the concept called, pipeline. Pipeline consists of,

  • Composition of stages.

  • Configurable transformation.

  • Stages can be arranged in multiple ways (with few eceptions).

• Pipeline are always an array of operations of one or more stages.

• Stages are composed of one or more aggregation operators or expressions.

  • The expression may take single or an array of arguments i.e expression dependent.

• Syntax

      db.collection_name.aggregate([{stage1},{stage2},...,{stageN}], {options})

• Example

        db.accounts.aggregate([
            {
                "$match": {"amenities":"Wifi"}
            }
            { 
                "$project": {"price": 1, "name": "Joe", "_id":0 }
            }
        ])
    

  • In the above example, we prepare an aggregation pipeline when we use aggregate function.

  • The first aggregation operator in pipeline is "$match" operation and then "$project" agggegation operation.

  • The aggregate function takes array as parameter. This array represents order of aggregation operation in pipeline as per which data need to be modified or organized.

  • The above query helps in filtering documents based on the match of field value and then only projecting based on field specified.

Aggregation Pipeline

• MQL can be considered as a subset of aggregation.

  • Read more about it here.

Some aggregation terms

• Field Path

  • Example

    • "$fieldName"

• System Variable

  • Example

    • "$$CURRENT"

      • All uppercase variables are system variable.

• User Variable

  • Example

    • "$$foo"

      • All lowercase variables are user variable.