Using node.js and mongoDB: Retrieve a custom value if the field is empty

My goal is to query the database for documents and if a certain field is not set, I want to replace it with a custom value. Let's say I have a collection of songs in my database where some have 'pathToCover' set and some don't. In those cases where it's not set, I'd like to return the URL to a placeholder image stored in a config variable.

Currently, I am using node.js and mongoDB with express and mongoose for this task. However, I'm unsure about the best approach to handle this situation. One solution could be querying for the documents and then checking each one in the callback function to see if the field is set, but that seems inefficient.

Here is an excerpt of my code:

exports.getByAlbum = function listByAlbum(query, callback) {
    Song.aggregate({ 
            $group: { 
                _id: '$album',
                artists: { $addToSet: '$artist' },
                songs: { $push: '$title' },
                covers: { $addToSet: '$pathToCover'},
                genres: { $addToSet: '$genre'},
                pathToCover: { $first: '$pathToCover'}
            }
        },
        function (err, result) {
            if (err) return handleError(err);

            result.forEach(function(album) {
                if ( album.pathToCover == null) {
                    album.pathToCover = config.library.placeholders.get('album');
                }
            })

            callback(result);
    });
}

I would appreciate any guidance on the optimal approach to solve this issue. Thank you!

node.jsmongodbmongoosemongodb-queryaggregation-framework

Answer №1

When the field value is either null or unset and potentially missing from the document, you can utilize $ifNull in your aggregation to assign an alternate value:

exports.getByAlbum = function listByAlbum(query, callback) {
  var defaultCover = config.library.placeholders.get('album');
  Song.aggregate(
    [
      { "$group": { 
        "_id": "$album",
        "artists": { "$addToSet": "$artist" },
        "songs": { "$push": "$title" }, 
        "covers": { "$addToSet": { "$ifNull": [ "$pathToCover", defaultCover ] } },
        "genres": { "$addToSet": "$genre" }, 
        "pathToCover": { "$first": { "$ifNull": [ "$pathToCover", defaultCover ] } }
      }}
    ],
    function (err, result) {
        if (err) return handleError(err);
        callback(result);
    }
  );
}

If it is an empty string, then use the $cond statement with an $eq test instead of $ifNull:

{ "$cond": [ { "$eq": [ "$pathToCover", "" ] }, defaultCover, "$pathToCover" ] }

You can use either statement within a grouping operator to substitute the considered value.

If you are concerned that not all values are set on the song, consider using $min or $max based on your data to select one value instead of $first.

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