[{"sub":{"country":"Netherlands","sex":"F","mWeight":60.9,"birthdate":ISODate("1961-05-26T00:00:00.000Z"),"age":38.5,},"process":[{"name":"A",},{"name":"B","measurements":{"m0010":1725.39,"m0020_mk":237.111,"m0021_I":168.385,"mvolume":61.5523}}]}]

db.collection.aggregate([{$unwind:"$process"},{$match:{"process.measurements":{$exists:true}}},{$group:{_id:"$_id",process:{$push:"$process"},sub:{$first:"$sub"}}},{$project:{sub:"$sub",measurements:{$arrayToObject:{$map:{input:"$process",as:"el",in:{"k":"$$el.name","v":"$$el.measurements"}}}}}},{$replaceRoot:{newRoot:{"$mergeObjects":[{"sub":"$sub"},"$measurements"]}}}])