[{"key":1,"someArray":[{field1:"value",field2:"value",Comment:""},{field1:"value",field2:"value",Comment:""},{field1:"value",field2:"value",Comment:"abc"}]},]

db.collection.aggregate([{"$project":{someArray:{$map:{input:"$someArray",as:"element",in:{$cond:{if:{$eq:["","$$element.Comment"]},then:{field1:"$$element.field1",field2:"$$element.field2"},else:"$$element"}}}}}},])