Question

In MVC3 Code First EF how do you associate one entity with another without creating a new one (many-to-many)?

So I have a many-to-many relationship between class1 and class2. I need class1 to hold many class2 and vice versa. However, class2 is independent; I have a list of them that I want to edit separately and then associate with a new class1.

When I pass my class2List to the controller( via AJAX and JSON), I checked and all the Ids of the class2s correspond to existing ids in the db, i.e. new class2s are not created.

Model

class
{
   [key]
   public int Id {set; get;} 
}
class1 : class 
{
        private ICollection<class2> _class2s;
        public virtual ICollection<class2> class2s
        {
            get { return _class2s ?? ( _class2s = new HashSet<class2>()); }
            set { _class2s = value; }
        }  
}

class2 : class
{
        private ICollection<class1> _class1s;
        public virtual ICollection<class1> class1s
        {
            get { return _class1s ?? ( _class1s = new HashSet<class1>()); }
            set { _class1s = value; }
        }  
}

Controller

public ActionResult SaveChanges(List<class2> class2List)
    {
        createNewClass2AndAssociateExistingClass2s(class2List);
        SaveChangesToDb();
        return View("ProductDetail", Model);
    }

createNewClass2AndAssociateExistingClass2s(List<class2> class2List)
{
    var newClass1 = newClass1()
    {
      class2s = class2List;
    }

    ////UnitOfWork allows me to access several DbSets in one transaction
    unitOfWork.Create(newClass1) 

}
SaveChangesToDb()
{
    unitOfWork.Commit();
}

What this does is create a new class1 (as it should) but instead of associating the existing class2s with it, it makes new class2s with new Ids and adds them to the database.

My question:

Does this have to do with how EF is reading my Id property from base class?

How would I be able to associate several existing class2s as a list with a new class1, without creating new class2s in the database?

Cheers

Answer 1

Ok so two things I learned from figuring this out:

• I was inheriting from an abstract class when I should have been implementing an interface. This is a great idea if you have several entities that have a similar property such as “Id” and you want to do something like

  T FindById<T>(int id) where T : IEntity

• When making associations in EF, even if the Id matches an existing entry, it will not update that entry, unless EF is tracking that entry in the context, as it says here. What I needed to do was:

  • Add a method in the mapping layer that gets the entry by id that I
    want from the repository
  • Copy the attributes of the new entry into that context entry
  • Return the context entry

Hope this helps someone