How to replace substring in mongodb document

db.media.find({mediaContainer:"ContainerS3"}).forEach(function(e,i) {    e.url=e.url.replace("//a.n.com","//b.n.com");    db.media.save(e);});

Nowadays,

• starting Mongo 4.2, db.collection.updateMany (alias of db.collection.update) can accept an aggregation pipeline, finally allowing the update of a field based on its own value.
• starting Mongo 4.4, the new aggregation operator $replaceOne makes it very easy to replace part of a string.

// { URL: "www.abc.com/helloWorldt/..." }// { URL: "www.abc.com/HelloWo/..." }db.collection.updateMany(  { URL: { $regex: /helloWorldt/ } },  [{    $set: { URL: {      $replaceOne: { input: "$URL", find: "helloWorldt", replacement: "helloWorld" }    }}  }])// { URL: "www.abc.com/helloWorld/..." }// { URL: "www.abc.com/HelloWo/..." }

• The first part ({ URL: { $regex: /helloWorldt/ } }) is the match query, filtering which documents to update (the ones containing "helloWorldt") and is just there to make the query faster.
• The second part ($set: { URL: {...) is the update aggregation pipeline (note the squared brackets signifying the use of an aggregation pipeline):
  • $set is a new aggregation operator (Mongo 4.2) which in this case replaces the value of a field.
  • The new value is computed with the new $replaceOne operator. Note how URL is modified directly based on the its own value ($URL).

Before Mongo 4.4 and starting Mongo 4.2, due to the lack of a proper string $replace operator, we have to use a bancal mix of $concat and $split:

db.collection.updateMany(  { URL: { $regex: "/helloWorldt/" } },  [{    $set: { URL: {      $concat: [        { $arrayElemAt: [ { $split: [ "$URL", "/helloWorldt/" ] }, 0 ] },        "/helloWorld/",        { $arrayElemAt: [ { $split: [ "$URL", "/helloWorldt/" ] }, 1 ] }      ]    }}  }])

Currently, you can't use the value of a field to update it. So you'll have to iterate through the documents and update each document using a function. There's an example of how you might do that here: MongoDB: Updating documents using data from the same document