[{"_id":1,"type":"big","fields":[11,12,13],"items":[21,22,23]},{"_id":2,"type":"small","fields":[14,15],"items":[24,25]},{"_id":3,"type":"small","fields":[],"items":[41,42]},{"_id":4,"type":"small","fields":[31,32,33],"items":[]}]

db.collection.aggregate([{$project:{types:[{type:"field",values:"$fields"},{type:"item",values:"$items"}]}},{$unwind:"$types"},{$project:{_id:0,value:{$arrayElemAt:["$types.values",0]},type:"$types.type",fromId:"$_id"}},{$match:{value:{$exists:true}}}])