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$replaceRoot (aggregation stage)

Definition

$replaceRoot

Replaces the input document with the specified document. The operation replaces all existing fields in the input document, including the _id field. You can promote an existing embedded document to the top level, or create a new document for promotion (see example).

Note

You can also use the $replaceWith stage. The $replaceWith stage peforms the same action as the $replaceRoot stage, but the stages have different forms.

The $replaceRoot stage has the following form:

{ $replaceRoot: { newRoot: <replacementDocument> } }

The replacement document can be any valid expression that resolves to a document. The stage errors and fails if <replacementDocument> is not a document. For more information on expressions, see Expressions.

Behavior

If the <replacementDocument> is not a document, $replaceRoot errors and fails.

If the <replacementDocument> resolves to a missing document (i.e. the document does not exist), $replaceRoot errors and fails. For example, create a collection with the following documents:

db.collection.insertMany([
   { "_id": 1, "name" : { "first" : "John", "last" : "Backus" } },
   { "_id": 2, "name" : { "first" : "John", "last" : "McCarthy" } },
   { "_id": 3, "name": { "first" : "Grace", "last" : "Hopper" } },
   { "_id": 4, "firstname": "Ole-Johan", "lastname" : "Dahl" },
])

Then the following $replaceRoot operation fails because one of the documents does not have the name field:

db.collection.aggregate([
   { $replaceRoot: { newRoot: "$name" } }
])

To avoid the error, you can use $mergeObjects to merge the name document into some default document; for example:

db.collection.aggregate([
   { $replaceRoot: { newRoot: { $mergeObjects: [ { _id: "$_id", first: "", last: "" }, "$name" ] } } }
])

Alternatively, you can skip the documents that are missing the name field by including a $match stage to check for existence of the document field before passing documents to the $replaceRoot stage:

db.collection.aggregate([
   { $match: { name : { $exists: true, $not: { $type: "array" }, $type: "object" } } },
   { $replaceRoot: { newRoot: "$name" } }
])

Or, you can use $ifNull expression to specify some other document to be root; for example:

db.collection.aggregate([
   { $replaceRoot: { newRoot: { $ifNull: [ "$name", { _id: "$_id", missingName: true} ] } } }
])

Examples

`$replaceRoot` with an Embedded Document Field

A collection named people contains the following documents:

{ "_id" : 1, "name" : "Arlene", "age" : 34, "pets" : { "dogs" : 2, "cats" : 1 } }
{ "_id" : 2, "name" : "Sam", "age" : 41, "pets" : { "cats" : 1, "fish" : 3 } }
{ "_id" : 3, "name" : "Maria", "age" : 25 }

The following operation uses the $replaceRoot stage to replace each input document with the result of a $mergeObjects operation. The $mergeObjects expression merges the specified default document with the pets document.

db.people.aggregate( [
   { $replaceRoot: { newRoot: { $mergeObjects:  [ { dogs: 0, cats: 0, birds: 0, fish: 0 }, "$pets" ] }} }
] )

The operation returns the following results:

{ "dogs" : 2, "cats" : 1, "birds" : 0, "fish" : 0 }
{ "dogs" : 0, "cats" : 1, "birds" : 0, "fish" : 3 }
{ "dogs" : 0, "cats" : 0, "birds" : 0, "fish" : 0 }

`$replaceRoot` with a Document Nested in an Array

A collection named students contains the following documents:

db.students.insertMany([
   {
      "_id" : 1,
      "grades" : [
         { "test": 1, "grade" : 80, "mean" : 75, "std" : 6 },
         { "test": 2, "grade" : 85, "mean" : 90, "std" : 4 },
         { "test": 3, "grade" : 95, "mean" : 85, "std" : 6 }
      ]
   },
   {
      "_id" : 2,
      "grades" : [
         { "test": 1, "grade" : 90, "mean" : 75, "std" : 6 },
         { "test": 2, "grade" : 87, "mean" : 90, "std" : 3 },
         { "test": 3, "grade" : 91, "mean" : 85, "std" : 4 }
      ]
   }
])

The following operation promotes the embedded document(s) with the grade field greater than or equal to 90 to the top level:

db.students.aggregate( [
   { $unwind: "$grades" },
   { $match: { "grades.grade" : { $gte: 90 } } },
   { $replaceRoot: { newRoot: "$grades" } }
] )

The operation returns the following results:

{ "test" : 3, "grade" : 95, "mean" : 85, "std" : 6 }
{ "test" : 1, "grade" : 90, "mean" : 75, "std" : 6 }
{ "test" : 3, "grade" : 91, "mean" : 85, "std" : 4 }

`$replaceRoot` with a newly created document

You can also create new documents as part of the $replaceRoot stage and use them to replace all the other fields.

A collection named contacts contains the following documents:

{ "_id" : 1, "first_name" : "Gary", "last_name" : "Sheffield", "city" : "New York" }
{ "_id" : 2, "first_name" : "Nancy", "last_name" : "Walker", "city" : "Anaheim" }
{ "_id" : 3, "first_name" : "Peter", "last_name" : "Sumner", "city" : "Toledo" }

The following operation creates a new document out of the first_name and last_name fields.

db.contacts.aggregate( [
   {
      $replaceRoot: {
         newRoot: {
            full_name: {
               $concat : [ "$first_name", " ", "$last_name" ]
            }
         }
      }
   }
] )

The operation returns the following results:

{ "full_name" : "Gary Sheffield" }
{ "full_name" : "Nancy Walker" }
{ "full_name" : "Peter Sumner" }

`$replaceRoot` with a New Document Created from `$$ROOT` and a Default Document

Create a collection named contacts with the following documents:

db.contacts.insertMany( [
   { "_id" : 1, name: "Fred", email: "fred@example.net" },
   { "_id" : 2, name: "Frank N. Stine", cell: "012-345-9999" },
   { "_id" : 3, name: "Gren Dell", home: "987-654-3210", email: "beo@example.net" }
] )

The following operation uses $replaceRoot with $mergeObjects to output current documents with default values for missing fields:

db.contacts.aggregate( [
   { $replaceRoot:
      { newRoot:
         { $mergeObjects:
             [
                { _id: "", name: "", email: "", cell: "", home: "" },
                "$$ROOT"
             ]
          }
      }
   }
] )

The aggregation returns the following documents:

{
  _id: 1,
  name: 'Fred',
  email: 'fred@example.net',
  cell: '',
  home: ''
},
{
  _id: 2,
  name: 'Frank N. Stine',
  email: '',
  cell: '012-345-9999',
  home: ''
},
{
  _id: 3,
  name: 'Gren Dell',
  email: 'beo@example.net',
  cell: '',
  home: '987-654-3210'
}

The C# examples on this page use the sample_mflix database from the Atlas sample datasets. To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see Get Started in the MongoDB .NET/C# Driver documentation.

The following Movie class models the documents in the sample_mflix.movies collection:

[BsonIgnoreExtraElements]
public class Movie
{
    [BsonId]
    public ObjectId Id { get; set; }
    [BsonElement("title")]
    public string Title { get; set; } = null!;
    [BsonElement("year")]
    public int? Year { get; set; }
    [BsonElement("runtime")]
    public int? Runtime { get; set; }
    [BsonElement("rated")]
    public string? Rated { get; set; }
    [BsonElement("metacritic")]
    public int Metacritic { get; set; }
    [BsonElement("plot")]
    public string? Plot { get; set; }
    [BsonElement("type")]
    public string? Type { get; set; }
    [BsonElement("cast")]
    public string[]? Cast { get; set; }
    [BsonElement("directors")]
    public string[]? Directors { get; set; }
    [BsonElement("writers")]
    public string[]? Writers { get; set; }
    [BsonElement("imdb")]
    public ImdbData? Imdb { get; set; }
}

The following ImdbData class models the embedded imdb document:

[BsonIgnoreExtraElements]
public class ImdbData
{
    [BsonElement("id")]
    public int? ImdbId { get; set; }
    [BsonElement("rating")]
    public double? Rating { get; set; }
    [BsonElement("votes")]
    public int? Votes { get; set; }
}

To use the MongoDB .NET/C# driver to add a $replaceRoot stage to an aggregation pipeline, call the ReplaceRoot() method on a PipelineDefinition object.

The following example creates a pipeline stage that filters for Movie documents that have an imdb field, sorts them by title, limits the result to five documents, and replaces each document with its embedded ImdbData document:

var pipeline = new EmptyPipelineDefinition<Movie>()
    .Match(Builders<Movie>.Filter.Exists(m => m.Imdb))
    .Sort(Builders<Movie>.Sort.Ascending(m => m.Title))
    .Limit(5)
    .ReplaceRoot(m => m.Imdb);

{ "rating" : "...", "votes" : 142 }
{ "rating" : "...", "votes" : 77 }
{ "rating" : "...", "votes" : 1608 }
{ "rating" : "...", "votes" : 2532 }
{ "rating" : "...", "votes" : 24645 }

The Node.js examples on this page use the sample_mflix database from the Atlas sample datasets. To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see Get Started in the MongoDB Node.js driver documentation.

To use the MongoDB Node.js driver to add a $replaceRoot stage to an aggregation pipeline, use the $replaceRoot operator in a pipeline object.

The following example creates a pipeline stage that replaces each input movie document with the document stored in its imdb property. The example then runs the aggregation pipeline:

const pipeline = [{ $replaceRoot: { newRoot: "$imdb" } }];
const cursor = collection.aggregate(pipeline);
return cursor;

Learn More

To learn more about related pipeline stages, see the $replaceRoot guide.

Back

Use Field Level Redaction

$replaceWith

Definition

Note

Behavior

Examples

$replaceRoot with an Embedded Document Field

$replaceRoot with a Document Nested in an Array

$replaceRoot with a newly created document

$replaceRoot with a New Document Created from $$ROOT and a Default Document

Learn More

`$replaceRoot` with an Embedded Document Field

`$replaceRoot` with a Document Nested in an Array

`$replaceRoot` with a newly created document

`$replaceRoot` with a New Document Created from `$$ROOT` and a Default Document