Populate limited number of items in array, but keep array length - mongoose

Question

With the schema

var CommentSchema = new Schema({
  text: { type: String },
  replies: [{ type: mongoose.Schema.ObjectId, ref: 'Comment' }]
});

I have a query like

Comment.findById(req.params.id)
  .populate({
    path: 'replies',
    model: 'Comment',
    options: {
      limit: 2
    }
  })
  .exec(...)

Now I see no option to populate a limited number of elements in an array and keep the length of the array.

I thought about populating to a different "target" field, so that the original replies-array remains untouched (and therefore the information about the number of replies). But I think this option doesn't exist in mongoose populate().

The use-case can be seen on Youtube comments. A comment includes the number of replies, but only displays a limited number of them.

Answer 1

I used the following trick to handle this situation.

var idToSearch = mongoose.Types.ObjectId(req.params.id)

var aggregation = [
    {$match : {_id : idToSearch}},
    {$project : {
      _id : 1,
      replies: {$slice : ['$replies',5]},
      totalreplies : {$size : "$replies"},
    }}
  ];
models.Comment.aggregate(aggregation)
    .exec(function(err, comments) {
      if(err){
        // return error 
      }
      else if(!comments){
        // return data not found
      }else {
        models.Comment.populate(comments,
          { path: 'replies'},
          function(err, populatedComments){
            if(err){
              // return error
            }
            else {
              console.log('comments ',populatedComments);
            }
          });
      }
    });

Hope it helps