You can use aggregate function to update MongoDB field using the value of another field. Here, we will create two collections:
name
studentInformation
<name> Collection
The query to create first collection with documents is as follows:
> db.name.insert({"FirstName":"John","LastName":"Smith"}); WriteResult({ "nInserted" : 1 })
Now you can display all documents from the collection with the help of find() method. The query is as follows:
> db.name.find().pretty();
The following is the output that displays the collection “name” documents:
{ "_id" : ObjectId("5c6c00dd68174aae23f5ef55"), "FirstName" : "John", "LastName" : "Smith" }
<studentInformation> Collection
The query to create second collection with documents is as follows:
> db.studentInformation.insert({"StudentFirstName":"Carol","StudentLastName":"Taylor"}); WriteResult({ "nInserted" : 1 })
Now you can display all documents from the collection with the help of find() method. The query is as follows:
> db.studentInformation.find().pretty();
The following is the output:
{ "_id" : ObjectId("5c6c013068174aae23f5ef56"), "StudentFirstName" : "Carol", "StudentLastName" : "Taylor" }
Now, let us update collection “name” with another collection “studentInformation”. The query is as follows:
> db.studentInformation.aggregate( [ {"$addFields":{"FullName":{"$concat":["$StudentFirstName"," ","$StudentLastName"]} }}, {"$out":"name"} ] );
Now you can check the documents of the collection “name”. The query is as follows:
> db.name.find().pretty();
The following is the output that displays that we have successfully updated the fields:
{ "_id" : ObjectId("5c6c013068174aae23f5ef56"), "StudentFirstName" : "Carol", "StudentLastName" : "Taylor", "FullName" : "Carol Taylor" }