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Limit Fields to Return from a Query¶
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The projection specification limits the fields to return for
all matching documents. The projection takes the form of a
document with a list of fields for inclusion or exclusion from
the result set. You can either specify the fields to include (e.g. {
field: 1 }) or specify the fields to exclude (e.g. { field: 0 }).
Important
The _id field is, by default, included in the result
set. To exclude the _id field from the result set, you need to
specify in the projection document the exclusion of the _id
field (i.e. { _id: 0 }).
You cannot combine inclusion and exclusion semantics in a single
projection with the exception of the _id field.
This tutorial offers various query examples that limit the fields to
return for all matching documents. The examples in this tutorial use a
collection inventory and use the db.collection.find()
method in the mongo shell. The
db.collection.find() method returns a cursor to the retrieved documents. For examples on query
selection criteria, see Query Documents.
Return All Fields in Matching Documents¶
If you specify no projection, the find() method returns all fields of all documents that
match the query.
This operation will return all documents in the inventory
collection where the value of the type field is 'food'. The
returned documents contain all its fields.
Return the Specified Fields and the _id Field Only¶
A projection can explicitly include several fields. In the following
operation, find() method returns all
documents that match the query. In the result set, only the item
and qty fields and, by default, the _id field return in the
matching documents.
Return Specified Fields Only¶
You can remove the _id field from the results by specifying its
exclusion in the projection, as in the following example:
This operation returns all documents that match the query. In the
result set, only the item and qty fields return in the
matching documents.
Return All But the Excluded Field¶
To exclude a single field or group of fields you can use a projection in the following form:
This operation returns all documents where the value of the type
field is food. In the result set, the type field does not
return in the matching documents.
With the exception of the _id field you cannot combine inclusion
and exclusion statements in projection documents.
Projection for Array Fields¶
The $elemMatch and $slice projection
operators are the only way to project portions of an array.
Tip
MongoDB does not support projections of portions of arrays
except when using the $elemMatch and $slice
projection operators.