MongoDB $lookup creates array

Question

So I am trying to join two collections together:

Collections are:

1. shows
2. episodes

I am using the $lookup value inside the shows collection.

[{$lookup: {
  from: 'episode',
  localField: 'url',
  foreignField: 'show_b',
  as: 'match_docs'
}}]

However I am getting all of the episodes from each show inside the match_docs in theory that is fine, however I need to be able to limit it to the latest episode limit:1 for each show ordered by pubDate

If anyone knows how I could limit the match_docs to only lookup once that would be great

I have also tried

{
  from: 'episode',
  localField: 'url',
  foreignField: 'show_b',
  pipeline: [
              {$sort:{id:1}},{$limit:1},
              ],
  as: 'match_docs'
}

With no success.

Answer 1

That would be easy with the second syntax of $lookup:

[
    {
        $lookup: {
            from: 'episodes', # make sure your collection name is correct
            let: {url: '$url'},
            pipeline: [
                {
                    $match: {
                        $expr: {
                            $eq: ['$show_b', '$$url']
                        }
                    }
                },
                {
                     $sort: {
                        pubDate: -1
                    }
                },
                {
                    $limit: 1
                } 
            ],
            as: 'match_docs'
        }
    }
]