Lazy Loading not working

Question

I'm having trouble using EF Code first 4.1 and am probably not understanding something.

My understanding is that by marking relationships (whether collections or single objects) as virtual, they will be lazy loaded on demand, so I could do something like:

var page = context.Pages.Where(xxxx);

var department = page.Department; //load this on demand?
var name = department.Name; //null reference exception

page.DepartmentId is populated correctly, however. In addition, manual loading the reference with context.Entry(page).Reference(p => p.Department).Load() works, but the whole point of having an object model is to not have to obsessively do that everywhere.

public class Page
{
  public int DepartmentId { get; set; }
  public virtual Department { get; set; }
}

public class Department
{
  public virtual ICollection<Page> Pages { get; set; }
}

context OnModelCreating

 modelBuilder.Entity<Page>().HasRequired(x => x.Department).WithMany(y => y.Pages).HasForeignKey(x => x.DepartmentId).WillCascadeOnDelete(false);

I've not disabled lazy loading, of course.

Answer 1

I figured out what was wrong in my situation. I marked the default constructor as private so ef was not generating the proxies for me.

thanks everyone.

Answer 2

there's probably something wrong on some place in your project. I created simple application based on your pieces and everything works fine. Try it and see, what's different.

First, my simple context:

class MyContext : DbContext
{
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<Page>().HasRequired(x => x.Department).WithMany(y => y.Pages).HasForeignKey(x => x.DepartmentId).WillCascadeOnDelete(false);
    }

    public IDbSet<Page> Pages { get; set; }
    public IDbSet<Department> Departments { get; set; }
}

Almost dummy initializer to put some data into database:

class TestInit : DropCreateDatabaseAlways<MyContext>
{
    protected override void Seed(MyContext context)
    {
        base.Seed(context);

        var newDepartment = context.Departments.Create();
        newDepartment.Name = "test";
        context.Departments.Add(newDepartment);

        var newPage = context.Pages.Create();
        newPage.Department = newDepartment;
        context.Pages.Add(newPage);

        context.SaveChanges();

        // leaving nothing in context
        var fullContext = (context as IObjectContextAdapter).ObjectContext;
        fullContext.Detach(newDepartment);
        fullContext.Detach(newPage);
    }
}

And your classes:

public class Page
{
    public int ID { get; set; }
    public int DepartmentId { get; set; }
    public virtual Department Department { get; set; }
}

public class Department
{
    public int ID { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Page> Pages { get; set; }
}

Now the test: Database.SetInitializer(new TestInit());

using (MyContext context = new MyContext())
{
    Console.WriteLine(context.Pages.Local.Count);
    Console.WriteLine(context.Departments.Local.Count);

    var page = context.Pages.First();
    Console.WriteLine(page.GetType().FullName);
    Console.WriteLine(page.Department.Name);
}

You should see (the proxy hash will be probably little bit different):

0
0
System.Data.Entity.DynamicProxies.Page_6D31E91F3B1E767826A19855D3A934B59F85AC161548E4761283C85824520E1D
test

First two zeros are there to be sure, that there are no entities in context. The type of Page class should be proxy, if it's the type from program itself, proxies are not being generated. Probably it's turned off or something is not virtual etc. Last line is actual name of department.

When I run the program, besides other queries, also:

SELECT 
[Extent1].[ID] AS [ID], 
[Extent1].[Name] AS [Name]
FROM [dbo].[Departments] AS [Extent1]
WHERE [Extent1].[ID] = @EntityKeyValue1

is sent to database.

Answer 3

Try the following model for Page and see if it helps

public class Page
{
    public int DepartmentId { get; set; }
    public virtual Department Department { get; set; }
}

You are missing the variable for Department. Hope it works now