cursos_grid.js

var instrutor = function(json, record) {
  var nome = "";
  record.instrutores.forEach(function(inst){ nome += (inst.nome + ", ") });
  return nome;
}

var CursoFactory = Ext.data.Record.create(["id", "nome", "descricao", 
{name:"instrutores", convert: instrutor}]);

var store = new Ext.data.Store({
    url: "/cursos.json",
    reader: new Ext.data.JsonReader({
        id: "id", root: "results", total: "total"
    }, CursoFactory)
})
store.load();

var grid = new Ext.grid.GridPanel({
  store: store
   , colModel: new Ext.grid.ColumnModel({
      columns: [
        {id:"id", header: "Id", width: 20, sortable: true, dataIndex: "id"}
        , {header: "Nome", width: 200, sortable: true, dataIndex: "nome"}
        , {header: "Descrição", width: 200, sortable: true, dataIndex: "descricao"}
        , {header: "Instrutores", width: 200, sortable: true, dataIndex: "instrutores"}
      ]
   })
   , sm: new Ext.grid.RowSelectionModel()
   , title: "Cursos", width: 600, height: 300, frame: true
   , listeners: {
        rowdblclick : function( grid, rowIndex, event ) {
            var json = grid.getSelectionModel().getSelected().json;
            var win = new Ext.Window({
                 modal:true, title: "Curso", width: 300, height: 200
            });
            win.show();
        }
   }
   , style: "margin: auto;"
});
grid.render("treinamento");