07‐Aggregation

Definitions

1. Aggregation: Collection and summary of data

2. Stage: One of the built-in methods that can be completed on the data, but does not permanently alter it

3. Aggregation pipeline: A series of stages completed on the data in order

Structure of an Aggregation Pipeline

db.collection.aggregate([
    {
        $stage1: {
            { expression1 },
            { expression2 }...
        },
        $stage2: {
            { expression1 }...
        }
    }
])

Using $match and $group Stages in a MongoDB Aggregation Pipeline

$match

The $match stage filters for documents that match specified conditions. Here's the code for $match:

{
  $match: {
     "field_name": "value"
  }
}

$group

The $group stage groups documents by a group key.

{
  $group:
    {
      _id: <expression>, // Group key
      <field>: { <accumulator> : <expression> }
    }
 }

$match and $group in an Aggregation Pipeline

The following aggregation pipeline finds the documents with a field named "state" that matches a value "CA" and then groups those documents by the group key "$city" and shows the total number of zip codes in the state of California.

db.zips.aggregate([
{   
   $match: { 
      state: "CA"
    }
},
{
   $group: {
      _id: "$city",
      totalZips: { $count : { } }
   }
}
])

Using $sort and $limit Stages in a MongoDB Aggregation Pipeline

$sort

The $sort stage sorts all input documents and returns them to the pipeline in sorted order. We use 1 to represent ascending order, and -1 to represent descending order.

{
    $sort: {
        "field_name": 1
    }
}

$limit

The $limit stage returns only a specified number of records.

{
  $limit: 5
}

$sort and $limit in an Aggregation Pipeline

The following aggregation pipeline sorts the documents in descending order, so the documents with the greatest pop value appear first, and limits the output to only the first five documents after sorting.

db.zips.aggregate([
{
  $sort: {
    pop: -1
  }
},
{
  $limit:  5
}
])

Using $project, $count, and $set Stages in a MongoDB Aggregation Pipeline

Review the following sections, which show the code for the $project, $set, and $count aggregation stages.

$project

The $project stage specifies the fields of the output documents. 1 means that the field should be included, and 0 means that the field should be supressed. The field can also be assigned a new value.

{
    $project: {
        state:1, 
        zip:1,
        population:"$pop",
        _id:0
    }
}

$set

The $set stage creates new fields or changes the value of existing fields, and then outputs the documents with the new fields.

{
    $set: {
        place: {
            $concat:["$city",",","$state"]
        },
        pop:10000
     }
  }

$count

The $count stage creates a new document, with the number of documents at that stage in the aggregation pipeline assigned to the specified field name.

{
  $count: "total_zips"
}