mongodb c# sort by field in a list of entries

Question

I have a collection like this (I removed fields that are not related to the question)

{
   _id:ObjectId('5dd7d946cd9c645f1cdc21ef'),
   Versions: [
      {
           "Barcode" : "200830001128132700636"
      },
       {
           "Barcode" : "200830001128132700637"
      }
   ]
},
{
   _id:ObjectId('5dd7d946cd9c645f1cdc21eg'),
   Versions: [
      {
           "Barcode" : "200830001128132700638"
      },
       {
           "Barcode" : "200830001128132700639"
      }
   ]
}

I need to find the greatest (max) barcode in the whole collection.

I tried with a code like this:

var options = new FindOptions<Document>
        {
            Limit = 1,
            Sort = Builders<Document>.Sort.Descending(d => d.Versions.Select(v => v.BarCode).Aggregate((v1, v2) => string.Compare(v1, v2) > 0 ? v1 : v2))
        };
using var results = await _context.DocumentiItems.FindAsync(FilterDefinition<Document>.Empty, options);

But I get ArgumentNullException, I think it's unable to traslate the expression with the aggregate.

Can you suggest me a better approach?, if possible I want to avoid the use of BSON strings and use only labmda expressions.

The type of is DocumentiItems is IMongoCollection<Document>

Answer 1

this can be easily achieved with the AsQueryable() interface like so:

            var result = collection.AsQueryable()
                           .SelectMany(i => i.Versions)
                           .OrderByDescending(v => v.Barcode)
                           .Take(1)
                           .Single();

here's a test program:

 using MongoDB.Entities; using MongoDB.Entities.Core; using System; using System.Linq; namespace StackOverflow { public class Item : Entity { public Version[] Versions { get; set; } } public class Version { public string Barcode { get; set; } } public class Program { private static void Main(string[] args) { new DB("test", "localhost"); var result = DB.Queryable<Item>() .SelectMany(i => i.Versions) .OrderByDescending(v => v.Barcode) .Take(1) .Single(); Console.WriteLine($"max barcode: {result.Barcode}"); Console.Read(); } } }