I have a MongoDB database on Atlas that keeps details of books I have read. There are two collections: an authors one and a books one.

[BsonIgnoreExtraElements]
public class Author
{
    [BsonId][BsonRepresentation(BsonType.ObjectId)] public ObjectId _id { get; set; }
    [BsonElement("first")] public string? first { get; set; }
    [BsonElement("last")] public string? last { get; set; }
    [BsonElement("author_id")] public BsonInt32? author_id { get; set; }
    [BsonElement("FullName")][BsonIgnoreIfNull] public string? FullName { get; set; }

}

[BsonIgnoreExtraElements]
public class Book
{
    [BsonId][BsonRepresentation(BsonType.ObjectId)] public ObjectId _id { get; set; }
    [BsonElement("title")] public string? title { get; set; }
    [BsonElement("title_id")] public BsonInt32? title_id { get; set; }
    [BsonElement("author")] public string? author { get; set; }
    [BsonElement("issue_date")] public string? issue_date { get; set; }
    [BsonElement("return_date")] public string? return_date { get; set; }
}

There are 485 authors and 1452 books. The database provides data for an Android app created using MAUI. One of the pages provides a list of which authors have the most books in the database. This uses a LINQ group query.

public async Task<List<Popular>> GetPopular()
{
    IMongoQueryable<Popular>? query = (from book in booksCollection.AsQueryable()
                                       group book by book.author into g
                                       let lastBook = g.OrderByDescending(b => b.return_date).FirstOrDefault()
                                       select new Popular
                                       {
                                           Author = g.Key,
                                           Count = g.Count(),
                                           ReturnDate = lastBook.return_date,
                                           Title = lastBook.title
                                       }).OrderByDescending(g => g.Count).ThenBy(g => g.Author).Take(10);
    List<Popular>? popular = await query.ToListAsync();
    return popular;
}

A lot of the time this runs correctly and produces the list. However it does crash on the query.ToListAsync statement at random.

**MongoDB.Driver.Linq.Linq3Implementation.Ast.Optimizers.AstGroupingPipelineOptimizer+UnableToRemoveReferenceToElementsException:**
 'Exception of type 'MongoDB.Driver.Linq.Linq3Implementation.Ast.Optimizers.AstGroupingPipelineOptimizer+UnableToRemoveReferenceToElementsException' was thrown.'

I’m using Visual Studio 17.11.4 with MongoDB.Driver 2.9 that has LINQ 3 as the default. Is there an easy way to convert the query to use Builders or BsonDocument, or is there an error in the LINQ I’ve used?