[{"dailyReadings":[{heartRate:52},{heartRate:52}]}]

db.collection.update({},[{$set:{dailyReadings:{$concatArrays:[{$slice:["$dailyReadings",0,{$subtract:[{$size:"$dailyReadings"},1]}]},[{$mergeObjects:[{$arrayElemAt:[{$slice:["$dailyReadings",-1]},0]},{newField:"1"}]}]]}}}])