Update array in MongoDB document by variable index?

To update array in MongoDB document by variable index, use the below syntax. Here, yourIndexValue in the index value, where yourIndexVariableName is the variable name for index −

var yourIndexVariableName= yourIndexValue,
anyVariableName= { "$set": {} };
yourVariableName["$set"]["yourFieldName."+yourIndexVariableName] = "yourValue";
db.yourCollectionName.update({ "_id":  yourObjectId}, yourVariableName);

Let us first create a collection with documents −

> db.updateByVariableDemo.insertOne({"StudentSubjects":["MySQL","Java","SQL Server","PL/SQL"]});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5cd553c37924bb85b3f4893a")
}

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

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

This will produce the following output −

{
   "_id" : ObjectId("5cd553c37924bb85b3f4893a"),
   "StudentSubjects" : [
      "MySQL",
      "Java",
      "SQL Server",
      "PL/SQL"
   ]
}

Following is the query to update array in MongoDB document by variable index −

> var indexValue = 1,
... valueToUpdate= { "$set": {} };
> valueToUpdate["$set"]["StudentSubjects."+indexValue] = "MongoDB";
MongoDB
> db.updateByVariableDemo.update({ "_id":  ObjectId("5cd553c37924bb85b3f4893a") }, valueToUpdate)
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

Let us display the documents once again −

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

This will produce the following output −

{
   "_id" : ObjectId("5cd553c37924bb85b3f4893a"),
   "StudentSubjects" : [
      "MySQL",
      "MongoDB",
      "SQL Server",
      "PL/SQL"
   ]
}