Mongo SMT dose not support `add.fields=patch`

Hi, background is I'm using mongo connector SMT, assuming we have a doc with three columns:{_id: 1, a: 1, b:1, c:1}

If I update the whole doc:update({"_id" : 1}, {"a": 2 }) -> final row: {_id:1, a:2}
The smt will give me payload as => {id:1, a:2}

If I just set one column:update({"_id" : 1}, {$set: {a:2}}) -> final row:{_id:1, a:2, b:1, c:1}
The smt will give payload as => {id:1, a:2}

So when I get a payload as {id:1, a:2}, how should I determine whether I should replace all the doc as {a:2} or I just set a-> 2 remaining all the other fields the same.

I tried to add the patch to add.fields to distinguish the unset/set operations but got 

java.lang.IllegalArgumentException: Unexpected field name: