使用DataAccessService MVVM Light更新数据库

Question

This has been bugging me for a while now, what I would like to be able to do is update an existing record on the database by making a change to the ObservableCollection or the record class SquirrelDataGridActiveView .
This is my DataAccessService:

public interface IDataAccessService
{
    ObservableCollection<SquirrelDataGridActiveView> GetEmployees();
    void UpdateRecord(SquirrelDataGridActiveView Emp);
    int CreateEmployee(SquirrelDataGridActiveView Emp);
}

/// <summary>
/// Class implementing IDataAccessService interface and implementing
/// its methods by making call to the Entities using CompanyEntities object
/// </summary>
public class DataAccessService : IDataAccessService
{
    Drive_SHEntities context;
    public DataAccessService()
    {
        context = new Drive_SHEntities();
    }
    public ObservableCollection<SquirrelDataGridActiveView> GetEmployees()
    {
        ObservableCollection<SquirrelDataGridActiveView> Employees = new ObservableCollection<SquirrelDataGridActiveView>();
        foreach (var item in context.SquirrelDataGridActiveViews)
        {
            Employees.Add(item);
        }
        return Employees;
    }

    public int CreateEmployee(SquirrelDataGridActiveView Emp)
    {
        context.SquirrelDataGridActiveViews.Add(Emp);
        context.SaveChanges();
        return Emp.ID;
    }

    public void UpdateRecord(SquirrelDataGridActiveView temp)
    {

    }
}

As you can see there is already a GetEmployees() method and a CreateEmployee() method however I'm finiding it very difficult to update the database with the new values.

Any suggestions would be much appreciated.

Answer 1

Looks like the problem maybe be in your EF layer. Do you have any unit tests to check that the creation of a new Employee record works properly? Can you break into the CreateEmployee and inspect the Emp.ID field? See if it's set to something. If it is, the creation of the new record works properly. One observation, if I may: your data access service does not need to return an ObservableCollection. I assume you already have an observable collection in your view-model class. If you want to keep alive the binding between the data grid in the view and the observable collection property in the view-model class, you should not reassign the OC property of the view-model class. The pattern you should follow is this:

• return an IEnumeration from your data access service's GetEmployees() method;
• Optional, but recommended is to make GetEmployees awaitable;
• in your view-model class, clear the content of the OC property and then add, one by one, all the items in the IEnumeration collection returned by your GetEmployees() method. This way your binding will continue to work and any updates in the database will be reflected in the view's data grid.

You can keep the OC return from your GetEmployees, but in your view- model you still need to transfer all the items, one by one, into the property that is, most likely, bound to the data grid in the view. If you just assign the GetEmployees() returned OC list to the the view-model's property, your binding will be gone.

Perhaps your data grid does not refresh properly and that's why you conclude that the database is not updated when in fact it is.

HTH, Eddie