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$replaceRoot (aggregation)

Definition

$replaceRoot

New in version 3.4.

Promotes a specified document to the top level and replaces all other fields. 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).

The $replaceRoot stage has the following form:

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

The replacement document can be any valid expression that resolves to a document.

For more information on expressions, see Expressions.

Behavior

$replaceRoot operations fail with an error if <replacementDocument> is not a document.

If the replacement document refers to a field in the input document that does not exist, the operation fails with an error. To ensure that the replacement document exists, use a $match stage first to check for existence before passing documents to the $replaceRoot stage (see example).

Examples

$replaceRoot with an embedded document

A collection named produce contains the following documents:

{
   "_id" : 1,
   "fruit" : [ "apples", "oranges" ],
   "in_stock" : { "oranges" : 20, "apples" : 60 },
   "on_order" : { "oranges" : 35, "apples" : 75 }
}
{
   "_id" : 2,
   "vegetables" : [ "beets", "yams" ],
   "in_stock" : { "beets" : 130, "yams" : 200 },
   "on_order" : { "beets" : 90, "yams" : 145 }
}

The following operation uses the $replaceRoot stage to promote the in_stock document to the top level, discarding the current top level fields.

db.produce.aggregate( [
   {
     $replaceRoot: { newRoot: "$in_stock" }
   }
] )

The operation returns the following documents:

{ "oranges" : 20, "apples" : 60 }
{ "beets" : 130, "yams" : 200 }

$replaceRoot with a $match stage

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, "hamsters" : 3 } }
{ "_id" : 3, "name" : "Maria", "age" : 25 }

In order to run an aggregation operation with a $replaceRoot stage to promote the pets field to the top level, you need to also include a $match stage to filter out any documents which do not contain a pets field.

db.people.aggregate( [
   {
     $match: { pets : { $exists: true } }
   },
   {
     $replaceRoot: { newRoot: "$pets" }
   }
] )

The operation returns the following results:

{ "dogs" : 2, "cats" : 1 }
{ "cats" : 1, "hamsters" : 3 }

$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 an array element

A collection named contacts contains the following documents:

{ "_id" : 1, "name" : "Susan",
  "phones" : [ { "cell" : "555-653-6527" },
               { "home" : "555-965-2454" } ] }
{ "_id" : 2, "name" : "Mark",
  "phones" : [ { "cell" : "555-445-8767" },
               { "home" : "555-322-2774" } ] }

The following operation promotes the embedded document with key cell to the top level:

db.contacts.aggregate( [
   {
      $unwind: "$phones"
   },
   {
      $match: { "phones.cell" : { $exists: true } }
   },
   {
      $replaceRoot: { newRoot: "$phones"}
   }
] )

The operation returns the following results:

{ "cell" : "555-653-6527" }
{ "cell" : "555-445-8767" }