Many-to-many relationship read and write - sql-server

Question

I just want to get some clarity about link table for a many-to-many relationship in a ASP.NET MVC project. When the Controller and the Views are created, it seems like there are not any code for read and write to the link table!? The only thing that is autogenrated is the table OrderEmployee.
If I have understood it right, for each order I create, I also need to add the ID of the Employee who handled it in the OrderEmployee table? And when I want to list the Oders and want to know each Employee who handled that Order, I need to read from the OrderEmployee table? I can't find any tutorials about how to read and write to and from a link table.
Do I have to add this read and write code on my own in the controller? Preciate if I can get some clarity about this!
public class Order
{
public int ID { get; set; }
public string Name { get; set; }
public int? ManufacturerID { get; set; }
public virtual Manufacturer Manufacturer { get; set; }
public virtual ICollection<Employee> Employees { get; set; }
}
public class Employee
{
public int ID { get; set; }
public string Name { get; set; }
public int EmployeeNumber { get; set; }
public virtual ICollection<Timeunit> Timeunits { get; set; }
public virtual ICollection<Order> Orders { get; set; }
}

Answer 1

For reading and writing in a many to many relationship is not really difficult but hard to get it right in the beginning.
You don't have to add anything in your models, they are right.
Writing
I'll show you with an example of how to add a single order to an employee with a dropdown list. The concept for add multiple orders to employees is nearly the same, you'll find a link at the bottom with a tutorial.
First of all you should create view models like this :
public class CreateEmployeeViewModel
{
public int ID { get; set; }
public string Name { get; set; }
public int EmployeeNumber { get; set; }
public int SelectedOrderID { get; set; }
public SelectList OrdersList { get; set; }
}
Then in your EmployeesControler :
// GET: Employees/Create
public ActionResult Create()
{
/* Add this */
CreateEmployeeViewModel model = new CreateEmployeeViewModel();
model.OrdersList = new SelectList(db.Orders.ToList(), "ID", "Name");
return View(model);
}
The first parameter will be the list of objects in which you want to select something (an Order in this case). The second parameter is the name of the propertie of your Order you want to pass to the view model (as SelectOrderID) and finally the third parameter is the value you want to display in the dropdownlist.
And in your Create.cshtml replace the first line with this :
#model TestStackOverflow.Models.CreateEmployeeViewModel
Next add the dropdown list in the Html.BeginForm :
<div class="form-group">
#Html.LabelFor(model => model.OrdersList, htmlAttributes: new { #class = "control-label col-md-2" })
<div class="col-md-10">
#Html.DropDownListFor(model => model.SelectedOrderID, Model.OrdersList, new { #class = "form-control" })
#Html.ValidationMessageFor(model => model.SelectedOrderID, "", new { #class = "text-danger" })
</div>
</div>
Now you should have this in the create view.
Then go back to your EmployeesControler and find the POST method for create and replace it with this :
[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create(CreateEmployeeViewModel model)
{
if (ModelState.IsValid)
{
Employee employee = new Employee()
{
Name = model.Name,
ID = model.ID,
EmployeeNumber = model.EmployeeNumber,
Orders = new List<Order>()
};
employee.Orders.Add(db.Orders.Where(x => x.ID == model.SelectedOrderID).FirstOrDefault());
db.Employees.Add(employee);
db.SaveChanges();
return RedirectToAction("Index");
}
return View(model);
}
It will create a new Employee based on the view model values and add an order to this Employee. Now if you check your OrderEmployees table you should see that a new entry is created when you're adding an Employee.
Reading
Reading the values is really simpler than writing.
If you want to read all the employees that are handling an order, just do it like this (I took Index.cshtml from the Orders to demonstrate it)
Add this at the top of your file:
#using YourNameSpace.Models
And now a little further modify the file. It should look like this:
#foreach (var item in Model) {
<tr>
<td>
#Html.DisplayFor(modelItem => item.Name)
</td>
<td>
#Html.DisplayFor(modelItem => item.ManufacturerID)
</td>
<td>
#foreach (Employee e in item.Employees)
{
#e.Name <br />
}
</td>
<td>
#Html.ActionLink("Edit", "Edit", new { id=item.ID }) |
#Html.ActionLink("Details", "Details", new { id=item.ID }) |
#Html.ActionLink("Delete", "Delete", new { id=item.ID })
</td>
</tr>
}
Here you can find a tutorial on how to use SelectList to handle one to many relationships and many to many relationships in your views.
Hope this answer will help you.