And Entry and Edit Navigation Property using Entity Framework 6 - entity-framework

Question

This is probably a duplicated question, but I cannot find the exact answer or documentation.
I have a two tables with one to many relationship. Each item has a category, and CategoryID is the foreign key.
public partial class Item
{
public long ItemID { get; set; }
public string ItemName { get; set; }
public int CategoryID { get; set; }
public virtual Category Category { get; set; }
}
public partial class Category
{
public int CategoryID { get; set; }
public string Name { get; set; }
public virtual ICollection<Item> Items { get; set; }
}
When I try to add an item, it looks like I always need to make sure category is null. If I forgot to set it to null, EntityFrame will create a new category even the old catagory exist.
For example:
My db is:
Item
Item #1: id: 1, name TV categoryId : 1
Item #2: id: 2, name DVD categoryId : 1
Item #3: id: 3, name Cat categoryId: 2
Categories
Category #1: id: 1 name: electronics
Category #2: id: 2 name: pets.
If my code is
using (var dbContext = new DBContext())
{
Category category = dbContext.Categories.FirstOrDefault(c=>c.CategoryID = 2);
Item item = new Item()
{
Name = "dog",
CategoryId = 2,
Category = category,
};
dbContext.Items.Add(item);
dbContext.SaveChanges();
}
This code will cause another new category with duplicated name "pet" created. I guess it makes sense. But I want to know is there a guideline of how/when to set to the collection object(Category object) or when should I set it to null? What's SOP when I need to update an existing item's category? If I simply use the following code:
using (var dbContext = new DBContext())
{
Item item = dbContext.Items.FirstOrDefault(i => i.ItemID == 1);
item.CategoryId = 2;
dbContext.SaveChanges();
}
This code will not work. should I manually set the item.Category property to null? Or there is a better formation in EF6?

Answer 1

change Item Class to this
public partial class Item
{
public long ItemID { get; set; }
public string ItemName { get; set; }
public int CategoryID { get; set; }
[ForeignKey="CategoryID"]
public virtual Category Category { get; set; }
}
then you are using [ForeignKey] annotation in your class that connect id to data, and when you are creating new data just put category Id not data.
Item item = new Item()
{
Name = "dog",
CategoryID = 2,
};