Single Datagridview Master-Details, no error but only shows "(Collection)"

Question

I am using SQLite and I would like to use Master-Detail in a single DataGridView. No error return, I check the Master Table ("ASM") and the Details Table ("PART") their data are good. However, my gridAsmPart only shows one row with the word (Collection) on column PART. Any help will be appreciated.

  public void LoadData()
    {
       var asm = new SQLiteDataAdapter("SELECT * FROM ASM", dB.connectionString);
       var part = new SQLiteDataAdapter("SELECT * FROM PART", dB.connectionString);

       var ds = new DataSet();

        asm.Fill(ds, "ASM");
        part.Fill(ds, "PART");

        DataRelation data_relation = new DataRelation(
            "ASM_PART",
            ds.Tables["ASM"].Columns["PART"],
            ds.Tables["PART"].Columns["PART"]);
        ds.Relations.Add(data_relation);

        gridAsmPart.DataSource = ds;
    }

Edit 1: It seems that if I use the old DataGrid, it works.