MVC4 Entity Framework 5 Many-To-Many Save Entity to Database

Question

Hi I've been stuck to long on this problem. I've looked at a lot of examples but i cant find what im looking for. Any help is appreciated.

I use the Code-First approach.

I've enabled migration and the database is fine [Student] - [StudentCourse] - [Course].

Scenario: I have two entites -> Student

public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual List<Course> Courses { get; set; }
}

And Course

public class Course
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public virtual List<Student> Students { get; set; }
}

Nothing fancy about that... Ive created a ViewModel ->

public class StudentCourseViewModel
{
    public int Id { get; set; }
    public string Name { get; set; }

    public List<Course> Courses { get; set; }
}

View:

@model Project.Web.Models.StudentCourseViewModel @{ ViewBag.Title = "Edit"; }

Edit

@using (Html.BeginForm()) { @Html.ValidationSummary(true)

<fieldset>
    <legend>Student</legend>

    @Html.HiddenFor(model => model.Id)

    <div class="editor-label">
        @Html.LabelFor(model => model.Name)
    </div>
    <div class="editor-field">
        @Html.EditorFor(model => model.Name)
        @Html.ValidationMessageFor(model => model.Name)
    </div>

    <div class="editor-label">
        @Html.LabelFor(model => model.Courses)
    </div>
      <div class="editor-field">
    @for (int i = 0; i < Model.Students.Count(); i++)
    {
        <div style="border: dotted 1px; padding: 5px; margin: 10px;">
            @Html.HiddenFor(s => s.Students[i].Id)

            @Html.LabelFor(s => s.Students[i].Name[i + 1])
            @Html.EditorFor(s => s.Students[i].Name)
        </div>
    }
</div>

    <p>
        <input type="submit" value="Save" />
    </p>
</fieldset> }

Controller Action:

    [HttpPost]
    public ActionResult Edit(CourseStudentViewModel model)
    {
        var course = db.Courses.Find(model.CourseId);
        course.Name = model.CourseName;
        course.Description = model.CourseDescription;
        course.Students = model.Students;

        if (ModelState.IsValid)
        {

            db.Entry(course).State = System.Data.EntityState.Modified;
            db.SaveChanges();

            return RedirectToAction("Index");
        }

        return View(model);
    }

(Maby this is were i go wrong...)

Anyway, I want to create a new Student with optional many courses (textboxes -> courseName)

How should i do this?

The main issue is that i always get null values (student is fine, List of courses = NULL) back from my view [httpPost]Create -action.

I'm in need of guidance how to make this approach possible.

Thx J!

Are the Courses present in your entity, just not in the ViewModel? Show the code for transforming your entity model into a view model. — Kevin Junghans

Kevin Junghans Kevin Junghans · Accepted Answer · 2013-02-12T15:22:32

Your entities are not setup correctly for a many-to-many relationship. You need another entity to handle the many-to-many mapping. It would look something like this.

public class StudentsToCourses
{
   public int StudentId {get; set;}
   public int CourseId {get; set;}

   public virtual Student Student {get; set;}
   public virtual Course Course {get; set;}
}

Then your student model should be changed to this.

public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual List<StudentsToCourses> Courses { get; set; }
}

And your coursed model changed to this.

public class Course
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public virtual List<StudentsToCourses> Students { get; set; }
}

You also need to setup the foreign key relationship using the Fluent API. It would look something like this.

public class StudentsToCoursesConfiguration : EntityTypeConfiguration<StudentsToCourses>
{
    internal StudentsToCoursesConfiguration ()
    {
        this.HasKey(p => new {p.StudentId, p.CourseId});
        this.HasRequired(p => p.Student)
            .WithMany(p => p.Courses)
            .HasForeignKey(p => p.StudentId);
        this.HasRequired(p => p.Course)
            .WithMany(r => r.Students)
            .HasForeignKey(p => p.CourseId);

    }
}

MVC4 Entity Framework 5 Many-To-Many Save Entity to Database

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1 Answers