If we try to instanciate a new model to update a single column, it will also update (and possibly override) all columns with a default value defined. (collections included)
The ValueManager should handle default values properly (previous_value should not be None I think).
The PR looks good, but I tried to add a test for the issue described and could not reproduce it. could you have a look? It'd be good to have a test for the reported issue in addition to the underlying previous_value behavior, and I can't get one to work.
With some help from Alan, I've fixed the test. If the integration tests run clean, and if it looks good to you, feel free to merge.