mongodb - $sort child documents only

Question

When i do a find all in a mongodb collection, i want to get maintenanceList sorted by older maintenanceDate to newest.

The sort of maintenanceDate should not affect parents order in a find all query

{
   "_id":"507f191e810c19729de860ea",
   "color":"black",
   "brand":"brandy",
   "prixVenteUnitaire":200.5,
   "maintenanceList":[
      {
         "cost":100.40,
         "maintenanceDate":"2017-02-07T00:00:00.000+0000"
      },
      {
         "cost":4000.40,
         "maintenanceDate":"2019-08-07T00:00:00.000+0000"
      },
      {
         "cost":300.80,
         "maintenanceDate":"2018-08-07T00:00:00.000+0000"
      }
   ]
}

Any guess how to do that ? Thank you

Answer 1

Whatever order the fields are in with the previous pipeline stage, as operations like $project and $group effectively "copy" same position.So, it will not change the order of your fields in your aggregated result.

And the sort of maintenanceDate through aggregation will not affect parents order in a find all query. So, simply doing this should work.

Assuming my collection name is example.

 db.example.aggregate([
  {
    "$unwind": "$maintenanceList"
  },
  {
    "$sort": {
      "_id": 1,
      "maintenanceList.maintenanceDate": 1
    }
  },
  {
    "$group": {
      "_id": "$_id",
      "color": {
        $first: "$color"
      },
      "brand": {
        $first: "$brand"
      },
      "prixVenteUnitaire": {
        $first: "$prixVenteUnitaire"
      },
      "maintenanceList": {
        "$push": "$maintenanceList"
      }
    }
  }
])

Output: