db.collection.aggregate()¶

On this page

Definition
Example

Definition¶

db.collection.aggregate(pipeline)¶

New in version 2.2.

Calculates aggregate values for the data in a collection. Always call the aggregate() method on a collection object.

Parameter	Type	Description
`pipeline`	document	A sequence of data aggregation processes. See the aggregation reference for documentation of these operators.

Returns:	A document with two fields: `result` which holds an array of documents returned by the pipeline `ok` which holds the value `1`, indicating success.
Throws:	exception – Changed in version 2.4: If an error occurs, the `aggregate()` helper throws an exception. In previous versions, the helper returned a document with the error message and code, and `ok` status field not equal to `1`, same as the `aggregate` command.

Example¶

Consider a collection named articles that contains documents of the following format:

copy

{
  title : "this is my title" ,
  author : "bob" ,
  posted : new Date () ,
  pageViews : 5 ,
  tags : [ "fun" , "good" , "sport" ] ,
  comments : [
               { author :"joe" , text : "this is cool" } ,
               { author :"sam" , text : "this is bad" }
  ],
  other : { foo : 5 }
}

The following aggregation pivots the data to group authors by individual tags:

copy

db.articles.aggregate(
  { $project : {
                 author : 1,
                 tags : 1,
               }
  },
  { $unwind : "$tags" },
  { $group : {
               _id : { tags : "$tags" },
               authors : { $addToSet : "$author" }
             }
  }
)

The aggregation pipeline begins with the collection articles and selects the author and tags fields using the $project pipeline operator. The $unwind operator produces one output document per tag. Finally, the $group pipeline operator groups authors by tags.

The operation returns the following document:

copy

{
   "result" : [
                {
                  "_id" : { "tags" : "good" },
                  "authors" : [ "bob", "mike", ... ]
                },
                {
                  "_id" : { "tags" : "fun" },
                  "authors" : [ "bob", "al" ]
                },

                ...

              ],
   "ok" : 1
}

The returned document contains two fields:

result field, which holds an array of documents returned by the pipeline, and
ok field, which holds the value 1, indicating success.

For more information, see Aggregation Concepts, Aggregation Reference, and aggregate.

← Collection Methods db.collection.count() →