[{"employee":"Joe","areAllAttributesMatched":false,"characteristics":[{"step":"A","name":"house","score":"1"},{"step":"B","name":"car"},{"step":"C","name":"job","score":"3"}]}]

db.collection.aggregate([{"$unwind":"$characteristics"},{"$match":{$expr:{$eq:[{"$ifNull":["$characteristics.score",null]},null]}}},{"$group":{_id:null,documents:{$push:{"employee":"$employee","name":"$characteristics.name","step":"$characteristics.step",}}}},{$project:{_id:false}}])