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db.collection.findOne()

On this page

  • Definition
  • Compatibility
  • Syntax
  • Behavior
  • Examples

MongoDB with drivers

This page documents a mongosh method. To see the equivalent method in a MongoDB driver, see the corresponding page for your programming language:

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Definition

db.collection.findOne(query, projection, options)

Returns one document that satisfies the specified query criteria on the collection or view.

The returned document may vary depending on the number of documents that match the query criteria, and the query plan used:

Number of matching documents
Query plan
Result
0
Any
The method returns null
1
Any
The method returns the document that satisfies the specified query criteria.
2 or more
Collection Scan
The method returns the first document according to the natural order. In capped collections, natural order is the same as insertion order.
2 or more
Index Scan
The method returns the first document retrieved from the index.

Important

If the query plan changes to use a different index, the method may return a different document. If your use case requires that a particular record is chosen consistently, you must use the options document to specify a sort. For details on using findOne() with an options document, see the example.

If you specify a projection parameter, findOne() returns a document that only contains the projection fields. The _id field is always included unless you explicitly exclude it.

Note

Although similar to the find() method, the findOne() method returns a document rather than a cursor.

Compatibility

You can use db.collection.findOne() for deployments hosted in the following environments:

  • MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

Syntax

The findOne() method has the following form:

db.collection.findOne( <query>, <projection>, <options> )

The findOne() method takes the following parameters:

Parameter
Type
Description
query
document
Optional. Specifies query selection criteria using query operators.
projection
document
Optional. Specifies the fields to return using projection operators. Omit this parameter to return all fields in the matching document. For details, see Projection.
options
document

Optional. Specifies additional options for the query. These options modify query behavior and how results are returned. To see available options, see FindOptions.

Behavior

Client Disconnection

Starting in MongoDB 4.2, if the client that issued db.collection.findOne() disconnects before the operation completes, MongoDB marks db.collection.findOne() for termination using killOp.

Projection

Important

Language Consistency

As part of making find() and findAndModify() projection consistent with aggregation's $project stage:

The projection parameter determines which fields are returned in the matching documents. The projection parameter takes a document of the following form:

{ field1: <value>, field2: <value> ... }
Projection
Description
<field>: <1 or true>
Specifies the inclusion of a field. If you specify a non-zero integer for the projection value, the operation treats the value as true.
<field>: <0 or false>
Specifies the exclusion of a field.
"<field>.$": <1 or true>

Uses the $ array projection operator to return the first element that matches the query condition on the array field. If you specify a non-zero integer for the projection value, the operation treats the value as true.

Not available for views.

<field>: <array projection>

Uses the array projection operators ($elemMatch, $slice) to specify the array elements to include.

Not available for views.

<field>: <$meta expression>

Uses the $meta operator expression to specify the inclusion of available per-document metadata.

Not available for views.

<field>: <aggregation expression>

Specifies the value of the projected field.

With the use of aggregation expressions and syntax, including the use of literals and aggregation variables, you can project new fields or project existing fields with new values.

  • If you specify a non-numeric, non-boolean literal (such as a literal string or an array or an operator expression) for the projection value, the field is projected with the new value, for example:

    • { field: [ 1, 2, 3, "$someExistingField" ] }

    • { field: "New String Value" }

    • { field: { status: "Active", total: { $sum: "$existingArray" } } }

  • To project a literal value for a field, use the $literal aggregation expression; for example:

    • { field: { $literal: 5 } }

    • { field: { $literal: true } }

    • { field: { $literal: { fieldWithValue0: 0, fieldWithValue1: 1 } } }

Embedded Field Specification

For fields in an embedded documents, you can specify the field using either:

  • dot notation, for example "field.nestedfield": <value>

  • nested form, for example { field: { nestedfield: <value> } }

_id Field Projection

The _id field is included in the returned documents by default unless you explicitly specify _id: 0 in the projection to suppress the field.

Inclusion or Exclusion

A projection cannot contain both include and exclude specifications, with the exception of the _id field:

  • In projections that explicitly include fields, the _id field is the only field that you can explicitly exclude.

  • In projections that explicitly excludes fields, the _id field is the only field that you can explicitly include; however, the _id field is included by default.

For more information on projection, see also:

Examples

With Empty Query Specification

The following operation returns a single document from the bios collection:

db.bios.findOne()

With a Query Specification

The following operation returns the first matching document from the bios collection where either the field first in the embedded document name starts with the letter G or where the field birth is less than new Date('01/01/1945'):

db.bios.findOne(
   {
     $or: [
            { 'name.first' : /^G/ },
            { birth: { $lt: new Date('01/01/1945') } }
          ]
   }
)

With a Projection

The projection parameter specifies which fields to return. The parameter contains either include or exclude specifications, not both, unless the exclude is for the _id field.

Specify the Fields to Return

The following operation finds a document in the bios collection and returns only the name, contribs and _id fields:

db.bios.findOne(
    { },
    { name: 1, contribs: 1 }
)

Return All but the Excluded Fields

The following operation returns a document in the bios collection where the contribs field contains the element OOP and returns all fields except the _id field, the first field in the name embedded document, and the birth field:

db.bios.findOne(
   { contribs: 'OOP' },
   { _id: 0, 'name.first': 0, birth: 0 }
)

With an Option

The following operation uses the sort option to return the first matching document from the sorted bios collection. In this example, the collection is sorted by birth in ascending order.

db.bios.findOne(
   { },
   { },
   { sort: { birth: 1 } }
)

The findOne Result Document

You cannot apply cursor methods to the result of findOne() because a single document is returned. You have access to the document directly:

var myDocument = db.bios.findOne();
if (myDocument) {
   var myName = myDocument.name;
   print (tojson(myName));
}

Use Variables in let Option

You can specify query options to modify query behavior and indicate how results are returned.

For example, to define variables that you can access elsewhere in the findOne method, use the let option. To filter results using a variable, you must access the variable within the $expr operator.

Create a collection cakeFlavors:

db.cakeFlavors.insertMany( [
   { _id: 1, flavor: "chocolate" },
   { _id: 2, flavor: "strawberry" },
   { _id: 3, flavor: "cherry" }
] )

The following example defines a targetFlavor variable in let and uses the variable to retrieve the chocolate cake flavor:

db.cakeFlavors.findOne(
   { $expr: { $eq: [ "$flavor", "$$targetFlavor" ] } },
   { _id: 0 },
   { let : { targetFlavor: "chocolate" }
} )

Output:

{ flavor: 'chocolate' }

To see all available query options, see FindOptions.

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