Merge two array fields in MongoDB?

To merge, use $setUnion operator. Let us first create a collection with documents −

> db.mergeTwoArrayFieldDemo.insertOne({"NaturalNumbers":[1,2,3,8,10,20,30],"WholeNumbers":[0,1,2,63,78,20,45]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd68e4057806ebf1256f11d") }

Following is the query to display all documents from a collection with the help of find() method −

> db.mergeTwoArrayFieldDemo.find().pretty();

This will produce the following output −

{    "_id" : ObjectId("5cd68e4057806ebf1256f11d"),    "NaturalNumbers" : [       1,       2,       3,       8,       10,       20,       30    ],    "WholeNumbers" : [       0,       1,       2,       63,       78,       20,       45    ] }

Following is the query to merge two array field in MongoDB.

> db.mergeTwoArrayFieldDemo.aggregate([    { "$project": {       "MergedArray": { "$setUnion": [ "$NaturalNumbers", "$WholeNumbers" ] }    }} ]);

This will produce the following output −

{ "_id" : ObjectId("5cd68e4057806ebf1256f11d"), "MergedArray" : [ 0, 1, 2, 3, 8, 10, 20, 30, 45, 63, 78 ] }