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"
}