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db.collection.aggregate([{/** group all docs by month*/$group:{_id:{$month:{$dateFromString:{dateString:"$date",format:"%Y-%m-%d"}}},data:{"$push":"$$ROOT"}}},{/** group all groups into a single doc*/"$group":{"_id":null,"groupData":{"$push":{/** k,v for $arrayToObject*/"k":"$_id","v":{"$sortArray":{"input":"$data","sortBy":{"date":1}}}}}}},{"$replaceWith":{"$arrayToObject":{"$map":{"input":{/** sort by month*/"$sortArray":{"input":"$groupData","sortBy":{"k":1}}},"in":{"$mergeObjects":["$$this",{/** rewrite k as string*/"k":{"$toString":"$$this.k"}}]}}}}}])