MongoDB rename database field within array

As mentioned in the documentation there is no way to directly rename fields within arrays with a single command. Your only option is to iterate over your collection documents, read them and update each with $unset old/$set new operations.

I had a similar problem. In my situation I found the following was much easier:

  1. I exported the collection to json:
mongoexport --db mydb --collection modules --out modules.json
  1. I did a find and replace on the json using my favoured text editing utility.

  2. I reimported the edited file, dropping the old collection along the way:

mongoimport --db mydb --collection modules --drop --file modules.json

For what it's worth, while it sounds awful to have to do, the solution is actually pretty easy. This of course depends on how many records you have. But here's my example:

db.Setting.find({ 'Value.Tiers.0.AssetsUnderManagement': { $exists: 1 } }).snapshot().forEach(function(item)
    for(i = 0; i != item.Value.Tiers.length; ++i)
        item.Value.Tiers[i].Aum = item.Value.Tiers[i].AssetsUnderManagement;
        delete item.Value.Tiers[i].AssetsUnderManagement;
    db.Setting.update({_id: item._id}, item);

I iterate over my collection where the array is found and the "wrong" name is found. I then iterate over the sub collection, set the new value, delete the old, and update the whole document. It was relatively painless. Granted I only have a few tens of thousands of rows to search through, of which only a few dozen meet the criteria.

Still, I hope this answer helps someone!

Edit: Added snapshot() to the query. See why in the comments.

You must apply snapshot() to the cursor before retrieving any documents from the database. You can only use snapshot() with unsharded collections.

From MongoDB 3.4, snapshot() function was removed. So if using Mongo 3.4+ ,the example above should remove snapshot() function.