Congratulations and thanks for share it.

var duplicateDepartment = context.Departments

.Include("Administrator")

.Where(d => d.PersonID == department.PersonID)

.AsNoTracking()

.FirstOrDefault();"

should be:

"var duplicateDepartment = db.Departments

.Include("Administrator")

.Where(d => d.PersonID == department.PersonID)

.AsNoTracking()

.FirstOrDefault();"

This tutorial and the previous one was excellent. It allowed me quick start in MVC as I am experienced in ASP.NET 2.0. However, I found a fatal flaw in implementation of function ValidateOneAdministratorAssignmentPerInstructor.

The flaw/issue: ValidateOneAdministratorAssignmentPerInstructor logic (not the function) does not work in the system. Its still possible to have an instructor assigned as administrator for 2 departments.

How it would happen: Its a chance event however the all the logical errors that can be introduced in a system must be avoided even by chance events if its avoidable at all by the way business functions.

How to reproduce:

I have introduced a Thread.Sleep statement in functionValidateOneAdministratorAssignmentPerInstructor to reproduce this error.

1. Clear all departments to have no administrators.

2. Change the function ValidateOneAdministratorAssignmentPerInstructor as below. This line also introduces a static variable to reproduce the issue.

static int runCount = 0;

private void ValidateOneAdministratorAssignmentPerInstructor(Department department)

{

if (department.InstructorID != null)

{

int count = runCount;

Department duplicateDepartment = db.Departments

.Include("Administrator")

.Where(d => d.InstructorID == department.InstructorID)

.AsNoTracking()

.FirstOrDefault();

//db.Entry(duplicateDepartment).Reference(x => x.Administrator).Load();

if (duplicateDepartment != null && duplicateDepartment.DepartmentID != department.DepartmentID)

{

System.Diagnostics.Debug.WriteLine("Adding validation error.");

var errorMessage = String.Format(

"Instructor {0} {1} is already administrator of the {2} department.",

duplicateDepartment.Administrator.FirstMidName,

duplicateDepartment.Administrator.LastName,

duplicateDepartment.Name);

ModelState.AddModelError(string.Empty, errorMessage);

System.Diagnostics.Debug.WriteLine("Added validation error.");

}

if ((runCount % 2) == 0)

{

System.Threading.Thread.Sleep(15000);

}

else

{

System.Threading.Thread.Sleep(15000);

}

}

}

3. Now run the app and open edit page of one department, say "Mathematics".

4. Now open another explorer tab or window and open edit page of another department, say "Engineering".

5. Choose same administrator/instructor name for both.

6. Submit both. Ensure that both department updates are submitted ASAP.

7. You will see that the second submission(most probably) coming with same admin selected for both departments in index page.

This issue can be solved easily by following true(or somewhat) OOP code by not having InstructorID field in Department model class. That means, the Instructor table would be created with Department_ID field by EF. This way you will get concurrency exception and the user is informed even though you may have a table that in database is not good as per db standards. Or you can keep the same structure and still overcome this by having a SP handle the the update that a DbContext(I am still not knowledgeable enough whether its possible; as the time I checked, it was not possible) would call and that SP would lock the Department table row and check if an admin is assigned to another department and then update that department row and then unlock. Since db lock is there, another process trying to update would wait or timeout.

However, this issue highlights how various logical errors can be introduced in a system using DbContext that has a ForeignKeyId field and also associated object unless the developer is aware of. That means, there could be many people having this issue without realizing that they may have. So, my question is how is EF with code first trying to solve this kind of issues? Kindly let me know as this is a major issue and if code first is used in a system with more than 10s of objects which are interrelated then this issue would definitely happen.

Thanks

Suresh

Thanks again for the great tutorial and the response.

{

var departments = unitOfWork.DepartmentRepository.Get(

orderBy: q => q.OrderBy(d => d.Name));

ViewBag.SelectedDepartment = new SelectList(departments, "DepartmentID", "Name", SelectedDepartment);

int departmentID = SelectedDepartment.GetValueOrDefault();

return View(unitOfWork.CourseRepository.Get(

filter: d => !SelectedDepartment.HasValue || d.DepartmentID == departmentID,

orderBy: q => q.OrderBy(d => d.CourseID),

includeProperties: "Department"));

}

should be

includedproperties:"Department"

I found it very easy to follow as a total novice thats done little more than experiment before. Thanks for the help!

@Xequence: I have found that using the intelisence system means that whatever you named a method (ect) doesn't really matter as it will come out right as long as you pay attension to the screen output as you type. In theory you could have called the 'includedProperties:' propertiesIWant: and the effect would still be the same.

I have been doing some studying on the subject of MVC3 Scaffolding and believe that the following link will be very usefull, I have put some of it to good use when working through this tutorial and saved myself some work! Always a good thing!

blog.stevensanderson.com/

It covers the Scaffolding features that you can control from the package manager in VS. I found it quite surprising just how much of the coding work can be done for you, in many cases writing a basic class file is all you need to get a complex site built in only a few steps!

Give it a go!

>>in Visual Studio. To see the entire query, you can copy the

>>variable value and paste it into a text editor:

One can also hover over the 'Value' and get a formatted tooltip. Just in case anyone here was unaware of this.

One question:

The navigation property always has a plural noun name and points to an entity set having a singular noun name e.g. Instructor.Courses -> Course and Instructor.Departments -> Department.

In the latter case, it's not obvious that the Instructor "directs" or "heads" the Department(s) which is why I might want to use verb names.

If the navigation property is renamed, will there be ill effects? Will scaffolding still work? Can metadata attributes be applied to assist?

Thank you and great work!

- Each of the 3 last bullets explaining the sequence of events leading to the "ObjectStateManager cannot track..." error contain references to "the Update method of the department repository" calling "Attach", even though DepartmentController.cs instantiates its own SchoolContext and uses it directly without a repository.

- Under the subtitle "Examining Queries Sent to the Database", instructions are to "set a breakpoint in GenericRepository.cs on the return query.ToList(); statement of the Get method". It worked as intended the first time around, but after the Index method of CourseController.cs was replaced by a version which passes "orderBy" to the Get() method of the Repository, the breakpoint didn't get hit anymore. Inserting breakpoints on both return statements in the Get() method made it work. Even so, the SQL query that was revealed was somewhat different from the one in the tutorial (all the rows starting [Project1] were missing, as was the ORDER BY clause). I guess this was because the breakpoint that was hit was for the statement "return orderBy(query).ToList();", i.e. orderBy had probably not yet been baked into the query.

===

In CourseController.cs, call the new method from the Details method, as shown in the following example:

public ActionResult Details(int id)

{

var query = "SELECT * FROM Course WHERE CourseID = @p0";

return View(unitOfWork.CourseRepository.GetWithRawSql(query, id).Single());

}

===

I did not find a "Details" method that is of type "ActionResult" within the existing code in CourseController.cs. I went ahead and replaced the "Details" method of type "ViewResult" with this code snippet, and it seems to work... but what's the difference between a "ViewResult" and an "ActionResult" and when would I know to use one vs. the other?

forums.asp.net/

(Point A)

Run the Department Edit page and try to change a department's administrator to an instructor who is already the administrator of a different department. You get the expected error message:

<Here is display a PrtScr of "English" Department Edit page showing that Fadi Fahkouri is already an administrator of Mathematics department.>

(Point B)

Now run the Department Edit page again (are you saying select "back To List" then select to "Edit" this exact same page again? Or are you saying clear the selection for Administrator and proceed to edit the Budget amount for this current record?) and this time change the Budget amount. When you click Save, you see an error page:

Uh... No... no I didn't actually... it merely accepted my change to the Budget field, then returned to the Department Index.

Then the next block of text tries to describe the sequence of events that led to this "error" that I didn't have occur...

•The Edit method calls the ValidateOneAdministratorAssignmentPerInstructor method, which retrieves all departments that have Kim Abercrombie as their administrator. That causes the English department to be read. Because that's the department being edited, no error is reported. As a result of this read operation, however, the English department entity is now being tracked by the database context.

Right off the bat... this blurb of text mentions a specific Instructor of "Kim Ambercombie". As best as I can figure, since I'm not achieving this error page talked about... I'm thinking there were perhaps some more specific instructions to carry-out for the user actions I was supposed to perform in sequence in order to make the error occur for me so that I could have the lesson trying to be taught go full-circle for me conceptually?