Introduction to Entity Framework Part 2

1. Introduction to Entity FrameworkPart 2 CRUD Scaffolding Tom Perkins NTPCUG

2. Quo Vadis • Previously (Part 01), we created an MVC application • We stored and displayed data using SQL Server LocalDB • This tutorial • Develop CRUD capabililty (Create, Read, Update, and Delete pages) • MVC scaffolding feature automatically creates basid code for you in Views and Controllers • Pages we’ll create follow …

3. Note the display of the courses for which the student is enrolled.

6. Objective 1 Display courses each student is enrolled in

7. Task: Display Student Courses Student Details (Target) Student Details (Current)

8. Modify Views\Student\Details.cshtml • Examine code in Views\Student\Details.cshtml

9. @model Statement • @model ContosoUniversity.Models.Studentindicates you want to use the ContosoUniversity.Models.Student object as data for this view • This object is created in the Controllers\StudentController.cs class – the id field is provided by the model binder using routing data. public ActionResult Details(int? id) { if (id == null) { return new HttpStatusCodeResult(HttpStatusCode.BadRequest); } Student student = db.Students.Find(id); if (student == null) { return HttpNotFound(); } return View(student); }

10. Open Views\Student\Details.cshtml • Each field is displayed using a DisplayFor helper. <dt> @Html.DisplayNameFor(model => model.LastName) </dt> <dd> @Html.DisplayFor(model => model.LastName) </dd>

11. Add code in Mod 1 Lazy Loading – a new query is generated each time you access the Enrollments navigation property.

12. Run the Application • (If you press CTRL+F5 while the Details.cshtml file is open, you'll get an HTTP 400 error because Visual Studio tries to run the Details page but it wasn't reached from a link that specifies the student to display. In that case, just remove "Student/Details" from the URL and try again, or close the browser, right-click the project, and clickView, and then click View in Browser.)

14. Objective 2 Update the ‘create’ page remove access to ‘ID’, add try-catch block

15. Module:Controllers\StudentController.cs • replace the HttpPost Create action method with the highlighted code in Modification 2

16. Walkthru: Modified Controllers\StudentController.cs • Read through code • The model binder • Coverts posted form values into CLR types (objects) • Passes them to an action method in parameters • Here, model binder creates a Student entity based on property values from the Form collection • Note: ID has been removed. ID set by SQL, not by user.

17. Security Note: •  The ValidateAntiForgeryToken attribute helps prevent cross-site request forgery (cookie modification) attacks. • It requires a correspondingHtml.AntiForgeryToken() statement in the view. • Bind attribute prevents overposting (i.e, Fiddler attack to modify a secret salary field.) Only fields listed in Bind are updated. • Try-Catch block could also log the error.

18. Walkthru\Views\Student\Create.cshtml • Note EditFor and ValidationMessageFor helpers instead of DisplayFor. • Also note @HtmlAntiForgeryToken() • Relevant code: <div class="form-group"> @Html.LabelFor(model => model.LastName, new { @class = "control-label col-md-2" }) <div class="col-md-10"> @Html.EditorFor(model => model.LastName) @Html.ValidationMessageFor(model => model.LastName) </div> </div>

19. Run, and try to enter Student data with invalid date

20. Run with corrected date

21. Objective 3 Update the Posting operation for the edit page

22. Module: Controllers\StudentController.cs • HttpGet Edit method does not need to be modified. • replace the HttpPostEdit action method with the code highlighted below in Modification 3 to add a try-catch block

23. Walkthru • Similar to Create changes • Difference: Entity not saved; it is marked as ‘Modified’. • SaveChanges() method will generate SQL statements to update row in table • All columns in row are updated, including those the user did not change • Currency conflicts are ignored

24. DbContext Maintains Entity State DbContext (In Sync?) Entities in Memory Rows in the Database Maintains Entity State SQL INSERT Add SaveChanges()

25. Entity States • Added • Entity doesn’t exist in database. • SaveChanges() issues a SQL INSERT query. • Unchanged • SaveChanges() – nothing is done • This is the initial state for an entity • Modified • Some or all property values have been changed • SaveChanges() issues an UPDATE query

26. Entity States, Continued • Deleted • Entity has been marked for deletion • SaveChanges() issues a DELETE command • Detached • Not being tracked by database context

27. Entity State Setting • Desktop Apps • Entity State is set automatically by Entity Framework • Web Apps • Disconnected nature • DbContext is disposed after page is rendered • Entity State must be set manually to ‘Modified’ • All columns in row will be updated • To update only columns modified by user, see more info on the Attach() method.

28. Edit a Student – Student tab, then Edit hyperlink

29. Change the Enrollment Date to 9/1/2011

30. Objective 4 Update the delete page - Add a custom error message when Savechanges() fails

31. DELETE Operations • Require 2 action methods • Give the user a chance to approve or disapprove the DELETE • If approved, a POST request is created • HttpPost Delete method is called • That method performs the Delete operation.

32. Delete Operation – Walkthru • Controllers\StudentController.cs – HttpGet Delete Action • Views\Student\Delete.cshtml • Controllers\StudentController.cs = HttpPostDeleteConfirmed Action Note the [HttpPost,ActionName(“Delete”)] attribute. This ensures the request generated by the Delete view will be routed to the DeleteConfirmed action.

33. Controllers\StudentController.cs – HttpGet Delete action • Apply the changes highlighted below to the HttpGet Delete action of Controllers\StudentController.cs -- code is in Modification 4.

34. Replace the HttpGetDeleteConfirmed Method [HttpPost] [ValidateAntiForgeryToken] public ActionResult Delete(int id) { try { Student student = db.Students.Find(id); db.Students.Remove(student); db.SaveChanges(); } catch (DataException/* dex */) { //Log the error (uncomment dex variable name and add a line here to write a log. return RedirectToAction("Delete", new { id = id, saveChangesError = true }); } return RedirectToAction("Index"); }

35. Add an Error Message • Add to \Views\Student\Delete.cshtml

36. Delete a Student …

38. Next – Paging, Filtering and Sorting FINIS