How to keep null values at the end of sorting in M

Question

This question already has an answer here:

• How can I sort into that nulls are last ordered in mongodb? 1 answer

I am querying a collection and sorting results based on a property. Some documents don't the that property value yet i.e. null. I want to keep the documents with null values at the end of the results after sorting and limiting (asc or desc). Is there a simple way in Mongoose to do that using a single query? If not, how can I use two queries separately since I have to limit the results as well for pagination?

var dealSchema = new Schema({

    // For Presentation Purposes Only
    dealId: { type: Number, index: true, unique: true },

    // BRAND Info
    brand: { type: ObjectId, ref: 'brand', index: true },
    brandUser: { type: ObjectId, ref: 'user', index: true, required: true },
    campaign: { type: ObjectId, ref: 'campaign', index: true },

    metrics: {
        totalCost: Number,
        totalValue: Number,
        roi: { type: Number, index: true },
        cpe: { type: Number, index: true }
    }

})

I want to sort based on 'metrics.cpe' in ascending order, but want null or 0 values to be at the end of the list. Example:

[0.1, 0.4, 0.7, 1.5, 2.5, 0, 0, null, null]

Sample document

{
  "_id": "590cdf29c102ae31208fa43a",
  "campaign": "587e6a880c844c6c2ea01563",
  "brand": "5a4cff5f8eaa1c26ca194acc",
  "brandUser": "57fd30cf0df04f1100153e07",
  "metrics": {
    "roi": 0,
    "cpe": 0,
    "totalValue": 0,
    "totalCost": 6000
  }
}

Answer 1

Am not sure about the solution am about to say. I cant test this out as I dont have a mongo db set right now, but I think that you can use <collection>.aggregate along with $project and $sort to achieve this.

Sample code:

db.inventory.aggregate(
   [
      {
         $project: {
            item: 1,
            description: { $ifNull: [ "$amount", -1*(<mimimum value>)* ] }
         }
      },
      { 
         $sort : { 
           amount : (-1 or 1 depending on the order you want)
         }
      }
   ]
)

Hope this helps !!