The onerous is mostly on the developer... For any "real" system that is going to be in production for a long time this becomes a real problem There are tools

The onerous is mostly on the developer... For any "real" system that is going to be in production for a long time this becomes a real problem There are tools to "migrate" data but they come with all the limitations of the Mongo isolation model Typically you either * Write ad hoc (possibly using some tooling) code to iterate over your old data adding or mutating the field(s) in question * Write queries such that they can handle the data being present, absent or in different forms for all of time. As you could expect this is a large burden