[{"_id":1,"staticData":{"dataType":"static","data":{"fred":1,"wilma":2}}},{"_id":2,"staticData":{"dataType":"dynamic","data":{"bambam":3,"dino":4}}},{"_id":3,"staticData":{}},{"_id":4}]

db.collection.aggregate([{"$set":{"staticData.data":{"$cond":[{"$eq":["$staticData.dataType","static"]},"$staticData.data",[]]}}},{"$project":{"staticData.data":1}}])