// There is no way to refer to a let variable ($$products) from a lambda function.
        // We have to use a BsonDocument (strings unfortunately) to define our lookup pipeline.
        private const string DeliveriesLookupPipelineString = @"
{$match:{
  $expr:{
    $and:[
      {$in:[""$productId"",""$$products.productId""]},
      {$eq:[""$deliveryType"",""Sample""]},
      {$eq:[""$availability"",""Released""]}
     ]
  }
 }
}
";

        private static readonly PipelineDefinition<Delivery, Delivery> DeliveriesLookupPipeline = new[]
        {
            BsonDocument.Parse(DeliveriesLookupPipelineString)
        };
...
                .Lookup<SearchAugmentedProductType, Delivery, Delivery, IEnumerable<Delivery>,
                    SearchAugmentedProductType>(
                    _deliveries,
                    new BsonDocument { { "products", "$products" } },
                    DeliveriesLookupPipeline,
                    searchAugmentedProductType => searchAugmentedProductType.Deliveries)
...