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- $project (aggregation)
$project (aggregation)¶
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Definition¶
-
$project
¶ Passes along the documents with only the specified fields to the next stage in the pipeline. The specified fields can be existing fields from the input documents or newly computed fields.
The
$project
stage has the following prototype form:The
$project
takes a document that can specify the inclusion of fields, the suppression of the_id
field, the addition of new fields, and the resetting the values of existing fields. The specifications have the following forms:Syntax Description <field>: <1 or true>
Specify the inclusion of a field. _id: <0 or false>
Specify the suppression of the _id
field.<field>: <expression>
Add a new field or reset the value of an existing field.
Considerations¶
Include Existing Fields¶
- The
_id
field is, by default, included in the output documents. To include the other fields from the input documents in the output documents, you must explicitly specify the inclusion in$project
. - If you specify an inclusion of a field that does not exist in the
document,
$project
ignores that field inclusion; i.e.$project
does not add the field to the document.
Suppress the _id
Field¶
The _id
field is always included in the output documents by
default. To exclude the _id
field from the output documents, you
must explicitly specify the suppression of the _id
field in
$project
.
Add New Fields or Reset Existing Fields¶
To add a new field or to reset the value of an existing field, specify the field name and set its value to some expression. For more information on expressions, see Expressions.
To set a field value directly to a numeric or boolean literal, as
opposed to setting the field to an expression that resolves to a
literal, use the $literal
operator. Otherwise,
$project
treats the numeric or boolean literal as a flag
for including or excluding the field.
By specifying a new field and setting its value to the field path of an existing field, you can effectively rename a field.
Embedded Document Fields¶
When projecting or adding/resetting a field within an embedded document, you can either use dot notation, as in
Or you can nest the fields:
When nesting the fields, you cannot use dot notation inside the
embedded document to specify the field, e.g. contact: {
"address.country": <1 or 0 or expression> }
is invalid.
Examples¶
Include Specific Fields in Output Documents¶
Consider a books
collection with the following document:
The following $project
stage includes only the _id
,
title
, and the author
fields in its output documents:
The operation results in the following document:
Suppress _id
Field in the Output Documents¶
The _id
field is always included by default. To exclude the _id
field from the output documents of the $project
stage,
specify the exclusion of the _id
field by setting it to 0
in
the projection document.
Consider a books
collection with the following document:
The following $project
stage excludes the _id
field but
includes the title
, and the author
fields in its output
documents:
The operation results in the following document:
Include Specific Fields from Embedded Documents¶
Consider a bookmarks
collection with the following documents:
To include only the title
field in the embedded document in the
stop
field, you can use the dot notation:
Or, you can nest the inclusion specification in a document:
Both specifications result in the following documents: