relational data in asp.net mvc - json

Question

I am getting 500 internal server error when method called for relational table data(Employee and department 0/1 to many ) to return as jsonresult.here is the method that gets error,
public JsonResult Index()
{
var employee = db.Employees.Include(x=>x.Department).ToList();
return Json(employee, JsonRequestBehavior.AllowGet);
}
but if i convert it as follows it works fine.
public JsonResult Index()
{ var emplist = db.Employees.ToList();
EmployeeViewModel emp = new EmployeeViewModel();
List<EmployeeViewModel> employee = emplist.Select(x => new EmployeeViewModel
{ EmployeeId = x.EmployeeId,
EmployeeName = x.EmployeeName,
DepartmentId = x.DepartmentId,
DepartmentName = x.Department.DepartmentName }).ToList();
return Json(employee, JsonRequestBehavior.AllowGet);
}
Is there any way I can get first method working..?

Answer 1

The error you are probably getting is due to MVC attempting to serialize your entity to pass to the client and the DB Context has fallen out of scope. (It is unclear how the DbContext is scoped with the code you have provided.) Serialization will iterate through every property in the entity, and for lazy-loaded references it will attempt to load them one by one. Even if the DbContext is scoped to the request and available this is very inefficient.
To avoid issues like this, simply do not pass EF entities between server and client. Nothing good will come of it. Pass a view model that represents just the data your view needs.
When you do:
var emplist = db.Employees.ToList();
// ^ Hits the database and loads ALL employees with all fields. (Does not load referenced data associated to employees, such as the departments.)
Employee emp = new Employee();
// ^ Does absolutely nothing for your cause.
List<Employee> employee = emplist.Select(x => new Employee
{ EmployeeId = x.EmployeeId,
EmployeeName = x.EmployeeName,
DepartmentId = x.DepartmentId,
DepartmentName = x.Department.DepartmentName }).ToList();
// ^ Selects 3 fields from each employee, then lazy-loads the department (works because the dbContext is in scope) and selects 1 field from the department. By using "new Employee" you are creating a POCO of the employee, not an EF proxy so the serializer will *not* attempt to resolve any dependencies.
return Json(employee, JsonRequestBehavior.AllowGet);
// ^ Serializes the POCO Employee object.
Instead, a better solution would be to declare an EmployeeViewModel that has EmployeeId, EmployeeName, DepartmentId, and DepartmentName, then use the following.
var employeeViewModels = db.Employees
.Select(x => new EmployeeViewModel
{
EmployeeId = x.EmployeeId,
EmployeeName = x.EmployeeName,
DepartmentId = x.Department.DepartmentId,
DepartmentName = x.Department.DepartmentName
}).ToList();
return Json(employeeViewModels, JsonRequestBehavior.AllowGet);
This will populate your view models with 1 hit to the database returning just the 4 fields you want to pass to the view rather than loading every field in the Employee plus an extra DB call to load the department. The view model is just a POCO so no weird behaviour around serialization. You can populate an Employee entity, however I'd recommend avoiding doing that because it can be confusing when working with an entity that may be an EF proxy (tripping up lazy loads) vs. a POCO entity which will be missing information and the EF context knows nothing about.