I have a doc in my collection which was inserted

collection.insert_one({'obj': {'a': 1, 'c': 3})

If I update it with

collection.update_one({}, {'$set': {'obj': {'a': 1, 'b': 2, 'c': 3}}})

I get {'obj': {'a': 1, 'b': 2, 'c': 3}} (same as doc update, as expected)

But if I update it with pipeline

collection.update_one({}, [{'$set': {'obj': {'a': 1, 'b': 2, 'c': 3}}}])

I get {'obj': {'a': 1, 'c': 3, 'b': 2}} (like there are not set of obj, but set of non exist b in obj)

If I need to update field of embedded doc, I use update with dot notation. But this full embedded doc update has unexpected behavior, therefore I created bug.

Also I chose Python Driver project because I found it with pymongo and motor, but It can be mongo server problem. If it is, move it please.