[{"_id":{"$oid":"649d0bae47fa0b4db69f2cf6"},"data":{"2":{"property":"str","comments":3},"3":{"property":"str","comments":3}}}]

db.collection.aggregate([{$project:{data:{$map:{input:{"$objectToArray":"$data"},in:{"$mergeObjects":[{key:"$$this.k"},"$$this.v"]}}}}}])