In MongoDb, with a Projection, remove array with one empty object [{}]

Question

In MongoDb, with a Projection, I want to remove an array that contains one empty object [{}] resulting from a $unwind preserveNullAndEmptyArrays & group.

[{
  "title": "Papaye",
  "childrens": [{}],
  "parents": [{
      "title": "Arbres fruitiers",
      "url": "/documents/plantes/arboriculture/arbres-fruitiers"
    }
  ],
  "url": "/documents/plantes/arboriculture/arbres-fruitiers/papaye"
},
{
  "title": "Arbres fruitiers",
  "childrens": [{
      "title": "Tavelure",
      "url": "/documents/maladies/tavelure"
    },
    {
      "title": "Longane",
      "url": "/documents/plantes/arboriculture/arbres-fruitiers/longane"
    }],
  "parents": [{
      "title": "Arboriculture",
      "url": "/documents/plantes/arboriculture"
     }],
 "url": "/documents/plantes/arboriculture/arbres-fruitiers"
}

Pipeline like :

var pipeline = [];
pipeline.push({$match:{url:/^\//}});
(...)
var proj = {};
proj.title = true;
proj.parents = true;
proj.url = true;

proj.parents = ???
proj.childrens = ???

pipeline.push({$project:proj});
db.getCollection('Pages').aggregate(pipeline)

Thanks in advance

tronic · Accepted Answer · 2017-09-13 07:20:52Z

2

Ok, I found the solution. Just test if url is null for the first array entry.

proj.childrens = {
    "$cond", [
                "$eq", [
                        "$arrayElemAt",["$childrens.url",0],
                        null
                    ],
                [],
                "$childrens"
              ]
};

answered Sep 13, 2017 at 7:20

tronic

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