Aggregation operations group values from multiple documents together, and can perform a variety of operations on the grouped data to return a single result. MongoDB provides three ways to perform aggregation: the aggregation pipeline, the map-reduce function, and single purpose aggregation methods.

when you need aggregation ?

• to perform group by
• to perform operations such as filter,sorting on the reference document
• to project the reference document fields in the resultset
• to paginate

Reference document ? What is it ?

Yes, In MongoDB you can refer one document from another using the value of the document’s _id field,collection name, and, optionally, its database name. DBRefs helps to acheive this operation.

DBRef documents resemble the following document:

{ "$ref" : <value>, "$id" : <value>, "$db" : <value> }

Consider a document from a collection that stored a DBRef in a creator field:

{
  "_id" : ObjectId("5126bbf64aed4daf9e2ab771"),
  // .. application fields
  "creator" : {
                  "$ref" : "creators",
                  "$id" : ObjectId("5126bc054aed4daf9e2ab772"),
                  "$db" : "users"
               }
}