This is a guideline to how make to complex linq for applied the group clause in the models.

complex_group_linq.cs

class TestModel
{
    public int columnA;
    public int columnB;
    public int columnC;
    public string columnD;
}

[HttpPost]
public JsonResult ModelLoad()
{
    List<TestModel> testModelList = new List<TestModel>();
    Dictionary<Tuple<int, int>, string> resultModel = new Dictionary<Tuple<int, int>, string>();

    for (int i=0; i<50; i++)
    {
        testModelList.Add(new TestModel()
        {
            columnA = i % 3,
            columnB = i,
            columnC = i % 4,
            columnD = string.Format("string {0}", i.ToString())
        });
    }

    testModelList.GroupBy(x => new
    {
        x.columnA,
        x.columnC
    })
    .ToList()
    .ForEach(g =>
    {
        var stringList = new List<string>();
        g
        .ToList()
        .ForEach(z =>
        {
            stringList.Add(z.columnB.ToString());
        });
        resultModel[Tuple.Create<int, int>(g.Key.columnA, g.Key.columnC)] = string.Join(", ", stringList);
    });

    return APPActor.GetJson(resultModel);
}