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"
   ]
}