How to remove properties from nested object array in MongoDB?

Question

I am trying to find the way to remove the properties from nested object array by using aggregate framework in MongoDB but I still can't find any solution for my issue. I have an nested object array like below:

{
  _id: '...',
  name: 'Warwick',
  description: 'Warwick desc...',
  reply: [
    {
      _id: '...',
      tag: 'champ',
      name: 'Warwick child',
      description: 'Warwick desc...',
      reply: [
         {
           _id: '...',
           tag: 'champ',
           name: 'Warwick child',
           description: 'Warwick desc...',
           reply: []
         },
         ...etc...
      ]
    },
    ...etc...
  ]
}

The desired results:

{
  _id: '...',
  name: 'Warwick',
  description: 'Warwick desc...',
  reply: [
    {
      _id: '...',
      name: 'Warwick child',
      description: 'Warwick desc...',
      reply: [
         {
           _id: '...',
           name: 'Warwick child',
           description: 'Warwick desc...',
           reply: []
         },
         ...etc...
      ]
    },
    ...etc...
  ]
}

What I need is remove tag property from each element in array. I tried using $unset but it doesn't seem to work in this case. How can I solve this problem?

Answer 1

I assume the depth of reply fields is not determined. I don't think you can do it purely in aggregation framework - but almost, using $function

Use $function and inside the body define a recursive function. Would be this one:

db.collection.aggregate([
   {
      $set: {
         reply: {
            $function: {
               body: function (doc, field) {
                  function removeField(obj) {
                     for (let obj in doc) {                         
                        for (let prop in obj) {
                           if (prop === field)
                              delete obj[prop];
                           else if (typeof obj[prop] === 'object')
                              removeField(obj[prop]);
                        }
                     }
                  }
                  removeField(doc);
                  return doc;
               },
               args: ["$reply", "tag"],
               lang: "js"
            }
         }
      }
   }
])