How Do I Populate Textboxes Based On Previious Textbox Value On Tabbing

See more:

ASP.NET

MVC

SQL-Server-2008R2

i want to populate second-textboxe based on first-textbox value on tabbing from database using jquery ajax in mvc4 e.g:
first-textbox is tasks now i want to populate second-textbox(projects)which is related to those tasks..i use this JSOn result function

public JsonResult LoadProjectsByTaskId(int  taskId)
        {
            var query = from tasks in context.Tasks
                        join projects in context.Projects on tasks.ProjectId equals                                     
                        projects.ProjectId
                        select new { tasks, projects };

            return Json(query, JsonRequestBehavior.AllowGet);
        }

and ajax code is

function LoadProjectsByTaskId() {

          $("#textarea").autocomplete({
              source: function (request, response) {
                  debugger;
                  $.ajax({
                      url: "/Home/LoadProjectsByTaskId",
                      type: "POST",
                      dataType: "json",
                      data: { term: request.term },
                      success: function (data) {
                          {
                              response($.map(data, function (item) {
                                  return { label: item.Name, value: item.Name };
                              }))
                           }
                      },
                  });
          }

      }

how can i do that?????