[{"key":1,"someArray":[{field1:"value",field2:"value",Comment:""},{field1:"value",field2:"value",Comment:""},{field1:"value",field2:"value",Comment:"abc"}]},{"key":1,"someArray":[{field3:"value",field4:"value",Comment:""},{field4:"value",field5:"value",Comment:""},{field8:"value",field9:"value",Comment:"abc"}]},]

db.collection.aggregate([{"$project":{someArray:{$map:{input:"$someArray",in:{$cond:{if:{$eq:["","$$this.Comment"]},then:{$arrayToObject:{$filter:{input:{$map:{input:{$objectToArray:"$$this"},as:"element",in:{$cond:[{$eq:["$$element.k","Comment"]},null,"$$element"]}}},as:"filter",cond:"$$filter"/** removes null's from array*/}}},else:"$$this"}}}}}}])