Insert Update Many to Many Entity Framework How do I do it

Question

I m using EF4 and new to it  I have a many to many in my project and cannot seem to work out how to insert or update  I have build a small project just to see how it should be coded   Suppose I have 3 tables   Class   ClassID-ClassName  Student   StudentID-FirstName-Surname StudentClass   StudentID-ClassID   After adding all the relationship and updated the model via model browser I have noticed that StudentClass does not appear  this seems to be default behaviour   Now I need to do both an Insert and Update  How do you do it  Any code samples or link where I can download an example  or can you spare 5 mins

User · Answer

I use the following way to handle the many-to-many relationship where only foreign keys are involved.

So for inserting:

public void InsertStudentClass (long studentId, long classId)
{
    using (var context = new DatabaseContext())
    {
        Student student = new Student { StudentID = studentId };
        context.Students.Add(student);
        context.Students.Attach(student);

        Class class = new Class { ClassID = classId };
        context.Classes.Add(class);
        context.Classes.Attach(class);

        student.Classes = new List<Class>();
        student.Classes.Add(class);

        context.SaveChanges();
    }
}

For deleting,

public void DeleteStudentClass(long studentId, long classId)
{
    Student student = context.Students.Include(x => x.Classes).Single(x => x.StudentID == studentId);

    using (var context = new DatabaseContext())
    {
        context.Students.Attach(student);
        Class classToDelete = student.Classes.Find(x => x.ClassID == classId);
        if (classToDelete != null)
        {
            student.Classes.Remove(classToDelete);
            context.SaveChanges();
        }
    }
}

User · Answer

In entity framework  when object is added to context  its state changes to Added  EF also changes state of each object to added in object tree and hence you are either getting primary key violation error or duplicate records are added in table

User · Answer

In terms of entities  or objects  you have a Class object which has a collection of Students and a Student object that has a collection of Classes  Since your StudentClass table only contains the Ids and no extra information  EF does not generate an entity for the joining table  That is the correct behaviour and that s what you expect   Now  when doing inserts or updates  try to think in terms of objects  E g  if you want to insert a class with two students  create the Class object  the Student objects  add the students to the class Students collection add the Class object to the context and call SaveChanges   using  var context   new YourContext          var mathClass   new Class   Name    Math         mathClass Students Add new Student   Name    Alice          mathClass Students Add new Student   Name    Bob           context AddToClasses mathClass       context SaveChanges        This will create an entry in the Class table  two entries in the Student table and two entries in the StudentClass table linking them together   You basically do the same for updates  Just fetch the data  modify the graph by adding and removing objects from collections  call SaveChanges  Check this similar question for details   Edit   According to your comment  you need to insert a new Class and add two existing Students to it   using  var context   new YourContext          var mathClass  new Class   Name    Math         Student student1   context Students FirstOrDefault s   gt  s Name     Alice        Student student2   context Students FirstOrDefault s   gt  s Name     Bob        mathClass Students Add student1       mathClass Students Add student2        context AddToClasses mathClass       context SaveChanges        Since both students are already in the database  they won t be inserted  but since they are now in the Students collection of the Class  two entries will be inserted into the StudentClass table

User · Answer

I wanted to add my experience on that  Indeed EF  when you add an object to the context  it changes the state of all the children and related entities to Added  Although there is a small exception in the rule here  if the children related entities are being tracked by the same context  EF does understand that these entities exist and doesn t add them  The problem happens when for example  you load the children related entities from some other context or a web ui etc and then yes  EF doesn t know anything about these entities and goes and adds all of them  To avoid that  just get the keys of the entities and find them  e g  context Students FirstOrDefault s   gt  s Name     Alice    in the same context in which you want to do the addition

User · Answer

Try this one for Updating    HttpPost  public ActionResult Edit Models MathClass mathClassModel          get current entry from db  db is context      var item   db Entry lt Models MathClass gt  mathClassModel          change item state to modified     item State   System Data Entity EntityState Modified         load existing items for ManyToMany collection     item Collection i   gt  i Students  Load           clear Student items               mathClassModel Students Clear           add Toner items     foreach  var studentId in mathClassModel SelectedStudents                var student   db Student Find int Parse studentId            mathClassModel Students Add student                              if  ModelState IsValid               db SaveChanges           return RedirectToAction  Index               return View mathClassModel

[c#] Insert/Update Many to Many Entity Framework . How do I do it?

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