$project

The projection stage takes a JSON document that can specify the inclusion of properties and/or the addition of new properties.

{"$project": {<projection spec>}}

Specifications

Specification	Description
property: 1 or true	Include this property in the projection
property: expression	Add a new property to the projection

Considerations

• Except for _id, only properties that you specify for inclusion or add to the projection as new properties will be projected on to the next stage.
• The _id property is always included in the projection, even if you don't specify inclusion.
• Existing properties cannot be overridden in a projection. If you wish to transform an existing property in a projection, it is recommended that you instead add a new property with a different name.

Examples

In the below example, a custom c_patient object is aggregated with a two stage pipeline in a script. The first stage is a $match that finds all patient records with c_gender: 'm'. That result set is then passed into the $project stage. The projection then specifies that c_name, and c_gender properties are included and that fullName and age are added as new properties.

• The new fullName property is a concatenation of the c_first and c_last sub-properties if the c_name document property using the $concat string operator and $string literal operator.
• The new age property is calculated from an existing c_birthdate property, using $subtract, $divide, and $floor arithmetic operators to calculate the age from the difference between c_birthdate and today's date.
• Since the $project stage is the last stage in the pipeline, the result of the projection is returned as the response.

Project Example

return org.objects.c_patients.aggregate()
  .match({
  	c_gender: 'm'
	})
	.project({
  	c_name: 1,
  	fullName: {$concat: ['c_name.c_first', {$string: ' '}, 'c_name.c_last']},
    c_gender: 1,
    age: { $floor: { $divide: [{ $subtract:[new Date(), 'c_birthdate']}, (365 * 24*60*60*1000)]}}
	})
	.toList();

Project Result

{
    "data": [
        {
            "_id": "59495fb2514ef1010000598b",
            "age": 38,
            "c_gender": "m",
            "c_name": {
                "c_first": "John",
                "c_last": "Smith"
            },
            "fullName": "John Smith"
        },
        {
            "_id": "59495fd7514ef1010000598f",
            "age": 48,
            "c_gender": "m",
            "c_name": {
                "c_first": "Charles",
                "c_last": "Jones"
            },
            "fullName": "Charles Jones"
        },
        {
            "_id": "59496012cfc40501006c10d2",
            "age": 43,
            "c_gender": "m",
            "c_name": {
                "c_first": "Richard",
                "c_last": "Roberts"
            },
            "fullName": "Richard Roberts"
        }
    ],
    "hasMore": false,
    "object": "list"
}