public void GetCategoriesByIdOnObjectContext(int categoryid)
{
//Arrange
var model = new NorthwindEntities();
ObjectContext context = model.ConvertContext();
//Act
var categories = context.CreateObjectSet <Category>("Categories");
var category = categories.SingleOrDefault(c => c.CategoryID == categoryid);
//Assert
Assert.IsNotNull(category);
Assert.AreEqual(categoryid, category.CategoryID);
}