`convertTo` schema `fillWhenMissing` option

[Jolanrensen]

Discussed in #1336

^{Originally posted by hantsy July 22, 2025}
How to apply the fill to the condition that the fields is missing the csv but defind in the DataSchema.

df.convertTo<MyData> {
     fill { col("NewCol") }.with{ "default" }
}

Currently, I faced the situation like this, we updated the Data Schema and add some new columns.

Some users use the new schema fields in the CSV file, and other not update and still used the old schema which lacked the new col here.

The fill is good to apply the default to old format CSV if they do not provide the new col. But the fill will override the value in the new format CSV file.

Is possible to only apply the fill to the old format here, and in the new format, use the value in CSV directly.

Adding a fillWhenMissing {} option would be a good solution for this