Database
[{"choiceA":[{"_id":ObjectId("5a934e000102030405000000"),"voter":"Juzi","choice":0,"pollId":100},{"_id":ObjectId("5a934e000102030405000001"),"voter":"Juma","choice":0,"pollId":100},{"_id":ObjectId("5a934e000102030405000002"),"voter":"Jane","choice":0,"pollId":100},]},{"choiceB":[{"_id":ObjectId("5a934e000102030405000003"),"voter":"Jamo","choice":1,"pollId":100},{"_id":ObjectId("5a934e000102030405000004"),"voter":"Juju","choice":1,"pollId":100},{"_id":ObjectId("5a934e000102030405000005"),"voter":"Jana","choice":1,"pollId":100}]},{"choiceC":[]}]
Query
db.collection.aggregate([{$group:{_id:null,x:{$push:"$$ROOT"}}},{$project:{_id:0,x:{$mergeObjects:"$x"}}},{$replaceRoot:{newRoot:"$x"}}])