How to move an array of embedded documents up to parent and change key/value with aggregation pipeline?

Use $replaceRoot in MongoDB aggregation. The $replaceRoot replaces the input document with the specified document. The operation replaces all existing fields in the input document, including the _id field. Let us create a collection with documents −

> db.demo733.insertOne( ...    { ...       "SubjectDetails": ...       [ ...          { ...             SubjectName:"MongoDB", ...             "Marks":85 ...          }, ...          { ...             SubjectName:"MySQL", ...             "Marks":90 ...          }, ...          { ...             SubjectName:"PL/SQL", ...             "Marks":98 ...          } ...       ] ...    } ... ); {    "acknowledged" : true,    "insertedId" : ObjectId("5eac6e6156e85a39df5f6342") }

Display all documents from a collection with the help of find() method −

> db.demo733.find();

This will produce the following output −

{ "_id" : ObjectId("5eac6e6156e85a39df5f6342"), "SubjectDetails" : [ { "SubjectName" : "MongoDB", "Marks" : 85 }, { "SubjectName" : "MySQL", "Marks" : 90 }, { "SubjectName" : "PL/SQL", "Marks" : 98 } ] }

Following is the query to move an array of embedded docs up to parent and change key/value with aggregation pipeline −

> db.demo733.aggregate([ ...    { ...       $replaceRoot: { ...          newRoot: { ...             $mergeObjects: [ ..               . { _id: "$_id" }, ...                { $arrayToObject: { $map: { input: "$SubjectDetails", in: [ "$$this.SubjectName", "$$this.Marks" ] } } } ...             ] ...          } ...       } ...    } ... ]).pretty()

This will produce the following output −

{    "_id" : ObjectId("5eac6e6156e85a39df5f6342"),    "MongoDB" : 85,    "MySQL" : 90,    "PL/SQL" : 98 }