Database

[{"_id":ObjectId("5eac6572fe57d55a258e645a"),"createdAt":ISODate("2020-05-01T18:07:37.568Z"),"user":ObjectId("5e6dda081c456d3670ba5816"),"question":[{"round":1,"attempt":1,"right":0,"wrong":1,"time":ISODate("2020-05-01T18:07:46.900Z"),"_id":ObjectId("5eac6572fe57d55a258e645b"),"questions":ObjectId("5e81735b286698630dba2be5")}],"__v":0},{"_id":ObjectId("5eac6572fe57d55a258e615a"),"createdAt":ISODate("2020-05-01T18:07:37.568Z"),"user":ObjectId("5e6dda081c456d3670ba5816"),"question":[{"round":1,"attempt":1,"right":0,"wrong":1,"time":ISODate("2020-05-01T18:07:46.900Z"),"_id":ObjectId("5eac6572fe57d55a258e645b"),"questions":ObjectId("5e81735b286698630dba2be5")},{"round":2,"attempt":2,"right":0,"wrong":2,"time":ISODate("2020-05-01T18:07:46.900Z"),"_id":ObjectId("5eac6572fe57d55a258e645b"),"questions":ObjectId("5e81735b286698630dba2be5")}],"__v":0},{"_id":ObjectId("5eac6572fe57d55a258e611a"),"createdAt":ISODate("2019-05-01T18:07:37.568Z"),"user":ObjectId("5e6dda081c456d3670ba5816"),"question":[{"round":1,"attempt":1,"right":0,"wrong":1,"time":ISODate("2020-05-01T18:07:46.900Z"),"_id":ObjectId("5eac6572fe57d55a258e645b"),"questions":ObjectId("5e81735b286698630dba2be5")},{"round":2,"attempt":2,"right":0,"wrong":2,"time":ISODate("2020-05-01T18:07:46.900Z"),"_id":ObjectId("5eac6572fe57d55a258e645b"),"questions":ObjectId("5e81735b286698630dba2be5")}],"__v":0},{"_id":ObjectId("5eac6572fe57d55a258e6123"),"createdAt":ISODate("2020-05-01T19:07:37.568Z"),"user":ObjectId("5e6dda081c456d3670ba5816"),"question":[{"round":1,"attempt":1,"right":0,"wrong":1,"time":ISODate("2020-05-01T18:07:46.900Z"),"_id":ObjectId("5eac6572fe57d55a258e645b"),"questions":ObjectId("5e81735b286698630dba2be5")},{"round":2,"attempt":2,"right":0,"wrong":2,"time":ISODate("2020-05-01T18:07:46.900Z"),"_id":ObjectId("5eac6572fe57d55a258e645b"),"questions":ObjectId("5e81735b286698630dba2be5")}],"__v":0}]

Query

db.collection.aggregate([{$addFields:{questionCount:{$size:"$question"}}},{$group:{_id:{createdAt:{$dateToString:{format:"%Y-%m-%d",date:"$createdAt"}}},questionCount:{$sum:"$questionCount"}}}])

Result