$replaceRoot (aggregation) |
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Docs Home → MongoDB Manual $replaceRoot (aggregation)On this page DefinitionBehaviorExamplesDefinition$replaceRootReplaces 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). NoteStarting in version 4.2, MongoDB adds a new aggregation pipeline stage, $replaceWith. 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: > } }The replacement document can be any valid expression that resolves to a document. The stage errors and fails if is not a document. For more information on expressions, see Expressions. BehaviorIf the is not a document, $replaceRoot errors and fails. If the 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 FieldA 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 ArrayA 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 documentYou 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 $$ROOT and a Default DocumentCreate a collection named contacts with the following documents: db.contacts.insertMany( [ { "_id" : 1, name: "Fred", email: "[email protected]" }, { "_id" : 2, name: "Frank N. Stine", cell: "012-345-9999" }, { "_id" : 3, name: "Gren Dell", home: "987-654-3210", email: "[email protected]" }] )The following operation uses $replaceRoot with $mergeObjects to output current documents with default values for missing fields: db.contacts.aggregate( [ { $replaceRoot: { newRoot: { $mergeObjects: [ { _id: "", name: "", email: "", cell: "", home: "" }, "$$ROOT" ] } } }] )The aggregation returns the following documents: { _id: 1, name: 'Fred', email: '[email protected]', cell: '', home: ''},{ _id: 2, name: 'Frank N. Stine', email: '', cell: '012-345-9999', home: ''},{ _id: 3, name: 'Gren Dell', email: '[email protected]', cell: '', home: '987-654-3210'}← $redact (aggregation)$replaceWith (aggregation) → |
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