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I'm using MongoDB v4.4. My document structure looks like this:

{
  _id: ObjectId("..."),
  photos: ["image1.png", "image2.png"]
},
{
  _id: ObjectId("..."),
  photos: ["image3.png", "another_image.png, image5.jpg"]
},
{
  _id: ObjectId("..."),
  photos: ["image_name.jpg", "image2.jpg"]
},

I am trying to change all the strings that contain ".png" to ".jpg".

I have tried the following:

db.users.updateMany({
  photos: { $regex: /.png/ }
}, [{
  $set: {
    "photos.$[]": { 
      $replaceOne: { 
        input: "photos.$[]", find: ".png", replacement: ".jpg" 
      } 
    }
  }
}])

This returns the error:

MongoServerError: Invalid $set :: caused by :: FieldPath field names may not start with '$'.

Answers

You need to iterate the loop of photos array and replace the extension,

• $map to iterate loop of photos array
• $replaceOne to replace extension

db.users.updateMany(
  { photos: { $regex: ".png" } },
  [{
    $set: {
      photos: {
        $map: {
          input: "$photos",
          in: {
            $replaceOne: {
              input: "$$this",
              find: ".png",
              replacement: ".jpg"
            }
          }
        }
      }
    }
  }]
)

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