hema g:

I am trying this mongo aggregation get the last four months of data and calculate the each month of avarage value i got the output but how to get null month of value (ex: 3 months data are there one month data are not inserted for example April month value not inserted but to display "April":0 this my requrement any one please suggest me.

db.collection.aggregate([
                            { $match: {
                                CREATE_DATE: {
                                    $lte:new Date(),
                                    $gte: new Date(new Date().setDate(new 
                                        Date().getDate()-120)
                                    )
                                }
                            } },
                            { $group: {
                                _id:{ 
                                    month: { $month: "$CREATE_DATE" },
                                    year: { $year: "$CREATE_DATE" }
                                },
                                avgofozone:{$avg:"$OZONE"}
                            } },
                            { $sort:{ "year": -1 } },
                            { $project: {
                                year: '$_id.year',
                                avgofozone: '$avgofozone',
                                month: '$_id.month',_id:0
                            } },
                            {
                        $addFields: { 
                            result: { 
                                $let: {
                                    vars: { "months": [ "Jan", "Feb", "Mar", "Apr", 
                         "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" ],


                                        arr: { $objectToArray: "$$ROOT" } }, 
                                    in: {
                                        "k" : { $arrayElemAt: [ "$$months" , { $subtract: [ { $arrayElemAt: [ "$$arr.v" , 2 ] }, 1 ] } ] }, // the key will be the month index translated into the string representation
                                        "v" : { $arrayElemAt: [ "$$arr.v" , 1 ] }
                                    }
                                }
                            }
                        }
                    }, {
                        $group: {
                            _id: "$year", // group by year
                            result: { $push: "$result" },
                        }
                    }, {
                        $addFields: {
                            "result": { $arrayToObject: "$result" },
                        }
                    }, {
                        $addFields: {
                            "result.year": "$_id"
                        }
                    }, {
                        $replaceRoot: {
                            newRoot: "$result"
                        }
                    }

                      ]
                       )
                    Actual Output:
                    {
                    "Mar" : 55.6,
                    "Jan" : 31.265,
                    "Feb" : 31.265, 
                    "year" : 2018
                 }
                   Expected Output:
                   {
                     "Mar" : 55.6,
                     "Jan" : 31.265,
                     "Feb" : 31.265,
                     "Apr" :0
                     "year" : 2018,

                  }