Download Introduction to Entity Framework Part 2

Introduction to Entity Framework
Part 2
CRUD Scaffolding
Tom Perkins
NTPCUG
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 …
Note the display of the courses for which the student is enrolled.
Objective 1
DISPLAY COURSES EACH STUDENT IS
ENROLLED IN
Task: Display Student Courses
Student Details (Target)
Student Details (Current)
Modify Views\Student\Details.cshtml
• Examine code in Views\Student\Details.cshtml
@model Statement
• @model ContosoUniversity.Models.Student indicates 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);
}
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>
Add code in Mod 1
Lazy Loading – a new query is
generated each time you access
the Enrollments navigation
property.
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.)
Objective 2
UPDATE THE ‘CREATE’ PAGE
REMOVE ACCESS TO ‘ID’, ADD TRY-CATCH BLOCK
Module:
Controllers\StudentController.cs
• replace the HttpPost Create action method
with the highlighted code in Modification 2
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.
Security Note:
• The ValidateAntiForgeryToken attribute helps
prevent cross-site request forgery (cookie
modification) attacks.
• It requires a
corresponding Html.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.
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>
Run, and try to enter Student data with invalid
date
Run with corrected date
Objective 3
UPDATE THE POSTING OPERATION
FOR THE EDIT PAGE
Module:
Controllers\StudentController.cs
• HttpGet Edit method does not need to be modified.
• replace the HttpPost Edit action method with the code
highlighted below in Modification 3 to add a try-catch block
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
DbContext Maintains Entity State
DbContext
(In Sync?)
Entities in Memory
Rows in the Database
Maintains
Entity State
SaveChanges()
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
Entity States, Continued
• Deleted
– Entity has been marked for deletion
– SaveChanges() issues a DELETE command
• Detached
– Not being tracked by database context
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.
Edit a Student – Student tab, then Edit
hyperlink
Change the Enrollment Date to
9/1/2011
Objective 4
UPDATE THE DELETE PAGE - ADD A CUSTOM
ERROR MESSAGE WHEN SAVECHANGES() FAILS
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.
Delete Operation – Walkthru
1. Controllers\StudentController.cs – HttpGet
Delete Action
2. Views\Student\Delete.cshtml
3. Controllers\StudentController.cs = HttpPost
DeleteConfirmed Action
Note the [HttpPost,ActionName(“Delete”)]
attribute. This ensures the request generated by
the Delete view will be routed to the
DeleteConfirmed action.
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.
Replace the HttpGet DeleteConfirmed
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");
}
Add an Error Message
• Add to \Views\Student\Delete.cshtml
Delete a Student …
Next – Paging, Filtering and
Sorting
FINIS