How do I push elements to an existing array in MongoDB?

To push elements to an existing array, use $addToSet operator along with update(). Let us first create a collection with documents −

> db.pushElements.insertOne({"Comments":["Good","Awesome","Nice"]}); {    "acknowledged" : true,    "insertedId" : ObjectId("5cd682597924bb85b3f48953") }

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

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

This will produce the following output −

{    "_id" : ObjectId("5cd682597924bb85b3f48953"),    "Comments" : [       "Good",       "Awesome",       "Nice"    ] }

Following is the query to push elements to an existing array in MongoDB −

> db.pushElements.update(    {_id:ObjectId("5cd682597924bb85b3f48953")},    { "$addToSet":{"Comments":"Cool"} },    upsert=true ); WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

Let us check the document once again −

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

This will produce the following output −

{    "_id" : ObjectId("5cd682597924bb85b3f48953"),    "Comments" : [       "Good",       "Awesome",       "Nice",       "Cool"    ] }