Mongo find next Tuesday-CodePudding

I'm trying to get the next Tuesday (day 2 of ISO Week) from today on. I have the current date from the global var "$$NOW" and the current day of ISO Week by the following operator: { $dayOfWeek: "$$NOW" }. But I can't think of any way to get the date of the next Tuesday in MongoDB-Aggregation.

For example i have as Input:

"$$NOW" which is => new Date("2021-09-13T16:20:00Z") // Today Monday

And now I want to get the next Tuesday:

new Date("2021-09-14T16:20:00Z") // coming Tuesday

which is quite easy, but if there is a week shift, it gets a bit more difficult. If the NOW date would be:

"$$NOW" which is => new Date("2021-09-16T16:20:00Z") // coming Thursday would be Today

the next Tuesday would be the:

new Date("2021-09-21T16:20:00Z") // Tuesday next week

So I always want to get the next following Tuesday after Today.

At the end, I want to create a mongo pipeline like this:

[{
    $addFields: {
        "nextTuesday": new Date(" the date of the next Tuesday ")
    }
}]

I would be very happy about any help!

CodePudding user response：

MongoDB has the $dayOfWeek operator that returns 1 for Sunday, 7 for Saturday.

On that scale, the formula to find a desired day of the week would be:

(((desired_day   6) - current_day) modulo 7)   1

Which in the MongoDB aggregation language, would look like:

{$addFields:{
    nextTuesday:{
     $toDate:{
      $add:[
         "$$NOW",
         {$multiply:[
             86400000,
             {$add:[
                  1,
                  {$mod:[
                     {$subtract:[
                           9,
                           {$dayOfWeek:"$$NOW"}
                     ]},
                     7
                  ]}
             ]}
         ]}
      ]
     }
    }
}}

Playground