NotNullFK.cs

static void Main(string[] args)
{
	Mapper.Initialize(config =>
	{
		config.CreateMap<Category, CategoryDto>();
		config.CreateMap<Article, ArticleDto>();
	});	
	Mapper.AssertConfigurationIsValid();
	
	var context = new AppDbContext();
	var articles = context.Articles.ProjectTo<ArticleDto>().ToList();
	articles.Dump();
}

class Initializer : DropCreateDatabaseAlways<AppDbContext>
{
	protected override void Seed(AppDbContext context)
	{
		var category = new Category { Name = "first" };
		context.Categories.Add(category);
		context.SaveChanges();
		context.Articles.AddRange(new[]{
			new Article { Content = "one", Title = "q1", Category = category },
			new Article { Content = "two", Title = "q2", Category = new Category { Name = "second" }},
			new Article { Content = "three", Title = "q3", Category = category },
		});
	}
}

public class AppDbContext : DbContext
{
	public AppDbContext()
	{
		Database.SetInitializer(new Initializer());
	}

	protected override void OnModelCreating(DbModelBuilder builder)
	{
		base.OnModelCreating(builder);

		builder.Entity<Article>()
			.HasRequired(a => a.Category)
			.WithMany(c => c.Articles)
			.HasForeignKey(n => n.CategoryId)
			.WillCascadeOnDelete();
	}

	public DbSet<Article> Articles { get; set; }

	public DbSet<Category> Categories { get; set; }
}

public class Article
{
	public int Id { get; set; }

	[Required, MaxLength(256)]
	public string Title { get; set; }

	[Required]
	public string Content { get; set; }

	public DateTimeOffset Created { get; set; }

	public Category Category { get; set; }
	public int CategoryId { get; set; }
}

public class ArticleDto
{
	public int Id { get; set; }
	public string Title { get; set; }
	public string Content { get; set; }
	public DateTimeOffset Created { get; set; }
	public CategoryDto Category { get; set; }
}

public class Category
{
	public Category()
	{
		Articles = new List<Article>();
	}
	
	public int Id { get; set; }

	[Required, MaxLength(256)]
	public string Name { get; set; }

	public List<Article> Articles { get; set; }
}

public class CategoryDto
{
	public int Id { get; set; }
	public string Name { get; set; }
}