Unable to update Foreign Key in Entity Framework 6

Since you are working with independent association. You can either

• Adding and removing the relationship from ContactList, but you need to retrieve from both Person.

  db.Entry(newContact).State = EntityState.Modified;
  
  var p1 = db.Set<Person>().Include(p => p.ContactList)
      .FirstOrDefault(p =>p.Id == 1);
  p1.ContactList.Remove(newContact);
  
  var p3 = db.Set<Person>().Include(p => p.ContactList)
      .FirstOrDefault(p => p.Id == 3);
  p3.ContactList.Add(newContact);
  
  db.SaveChanges();
  

• Or you can use disconnected object, but you need to manually manage the relationship.

  db.Entry(newContact).State = EntityState.Modified;
  
  var p1 = new Person { Id = 1 };
  db.Entry(p1).State = EntityState.Unchanged;
  var p3 = new Person { Id = 3 };
  db.Entry(p3).State = EntityState.Unchanged;
  
  var manager = ((IObjectContextAdapter)db).ObjectContext.ObjectStateManager;
  manager.ChangeRelationshipState(newContact, p1, item => item.ContactOwner,
       EntityState.Deleted);
  manager.ChangeRelationshipState(newContact, p3, item => item.ContactOwner,
       EntityState.Added);
  
  db.SaveChanges();
  

PS

You might need to reconsider adding foreign key value, to make everything easier, updating foreign key just by mentioning the Id.

See this post for more information.

This is what I ended up building till the late hours of the morning, chould/should be refactored some more...

   protected static async Task<int> SaveEntity<t>(t obj) where t : BaseModel
    {
        try
        {
            using (DatabaseContext db = GetDbContext())
            {

                //get the basemodel/fk reference properties
                IEnumerable<PropertyInfo> props = obj.GetType().GetProperties().Where(p => p.PropertyType.BaseType == typeof(BaseModel));

                if (obj.Id <= 0)
                {//insert

                    db.Entry(obj).State = EntityState.Added;

                    //set fk reference props to unchanged state
                    foreach (PropertyInfo prop in props)
                    {
                        Object val = prop.GetValue(obj);
                        if (val != null)
                        {
                            db.Entry(val).State = EntityState.Unchanged;
                        }
                    }

                    //do insert
                    return await db.SaveChangesAsync();

                }
                else
                {//update

                    //get the posted fk values, and set them to null on the obj (to avaid dbContext conflicts)
                    Dictionary<string, int?> updateFKValues = new Dictionary<string, int?>();
                    foreach (PropertyInfo prop in props)
                    {
                        BaseModel val = (BaseModel)prop.GetValue(obj);
                        if (val == null)
                        {
                            updateFKValues.Add(prop.Name, null);
                        }
                        else
                        {
                            updateFKValues.Add(prop.Name, val.Id);
                        }

                        prop.SetValue(obj, null);
                    }

                    //dbContext creation may need to move to here as per below working example
                    t dbObj = (t)db.Set(typeof(t)).Find(new object[] { obj.Id }); //this also differs from example

                    //update the simple values
                    db.Entry(dbObj).CurrentValues.SetValues(obj);

                    //update complex values
                    foreach (PropertyInfo prop in props)
                    {
                        Object propValue = null;
                        if (updateFKValues[prop.Name].HasValue)
                        {
                            propValue = (BaseModel)db.Set(prop.PropertyType).Find(new object[] { updateFKValues[prop.Name] });
                        }

                        prop.SetValue(dbObj, propValue);
                        if (propValue != null)
                        {
                            db.Entry(propValue).State = EntityState.Unchanged;
                        }

                    }

                    //do update
                    return await db.SaveChangesAsync();

                }

            }
        }
        catch (Exception ex)
        {
            ExceptionHelper.Log(ex);
            throw;
        }
    }