aggregate - reverse - lookup

// reverse lookup: https://stackoverflow.com/questions/63364037/mongoose-aggregation-reverse-lookup-or-lookup-without-localfield

const tags = await TagSchema.aggregate([
     {
         from: 'posts',
         let: { tagId: '$_id' },
         pipeline: [
             {
                 $match: {
                     $expr: {
                         $eq: ['$_id', '$$tagId'],
                     },
                 },
             },
             {
                 $project: {},
             },
         ],
         as: 'posts',
     },
    {
        $unwind: {
            path: '$posts',
            preserveNullAndEmptyArrays: true,
        },
    },
]);