$project or $group does not support

Question

I'm trying to run aggregate with projection but i get NotSupportedException: $project or $group does not support <document>. I am running version 2.4.4 of driver with mongodb v3.4.

var filter = Builders<T>.Filter.Regex(x=>x.Value,"/test/gi");

var aggregate = collection.Aggregate()
       .Match(filter)
       .Project(x => new 
       {
          Idx = x.Value.IndexOf("test"),
          Result = x
       })
       .SortBy(x => x.Idx);

I thought IndexOfCP is supported.

What am i doing wrong here?

Answer 1

The problem is caused not by IndexOf but by your projection. Projection should not include document itself, this just is not supported by MongoDB .Net driver. So the following query without including x object into projection will work just fine:

var aggregate = collection.Aggregate()
       .Match(filter)
       .Project(x => new 
       {
          Idx = x.Value.IndexOf("test"),
          // Result = x
       })
       .SortBy(x => x.Idx);

There are several possible fixes here. The best choice is to include in projection not the whole document but only the fields that are actually required for further logic, e.g.:

var aggregate = collection.Aggregate()
    .Match(filter)
    .Project(x => new
    {
        Idx = x.Value.IndexOf("test"),
        Value = x.Value,
        // ...
    })
    .SortBy(x => x.Idx);

If however you need the document object itself, you could fetch the whole collection to the client and then use LINQ to objects:

var aggregate = collection.Aggregate()
    .Match(filter)
    .ToList()
    .Select(x => new
        {
            Idx = x.Value.IndexOf("test"),
            Result = x
        })
        .OrderBy(x => x.Idx);

Use this approach as last option because it loads heavily both server and client.