使用外键添加实体对象

Question

In database I have 2 tables:
Article: ID | Title| SectionID
Section: ID | SectionName
(all ids set as Identity Increment)

I want to add new Article with specific SectionID (that exits in database).

I tired this:

Article art = new Article
{
    Title = "title"
};

art.Section.SectionID = 0;


if (art.EntityState == EntityState.Detached)
{
   Articlerctx.AddToArticles(art);
}

I get error on art.Section.SectionID = 0;

Object reference not set to an instance of an object

Do I need to create a complete object just for one int field???

Answer 1

You can of course get the object from the database (I didn't see the "exists in database", sorry).

just grab the Section object from your context and assign it.

art.Section = context.Section.FirstOrDefault(s => s.ID == 0);

Answer 2

art.SectionReference.EntityKey = new EntityKey("EntityModelName.EntitySetName", "IdFieldName", 0);

Answer 3

In EF 4 you can check the box on the update model wizard to include foreign key fields in the model. If you do this, you can then change your code to:

Article art = new Article
{
    Title = "title"
};

art.SectionID = 0;

if (art.EntityState == EntityState.Detached)
{
   Articlerctx.AddToArticles(art);
}