One-to-One relationships in Entity Framework 7 Code First

Question

How to configure One-to-One or ZeroOrOne-to-One relationships in Entity Framework 7 Code First using Data Annotations or Fluent Api?

Answer 1

You can define OneToOne relationship using Fluent API in Entity Framework 7 as below

class MyContext : DbContext
{
    public DbSet<Blog> Blogs { get; set; }
    public DbSet<BlogImage> BlogImages { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Blog>()
            .HasOne(p => p.BlogImage)
            .WithOne(i => i.Blog)
            .HasForeignKey<BlogImage>(b => b.BlogForeignKey);
    }
}

public class Blog
{
    public int BlogId { get; set; }
    public string Url { get; set; }

    public BlogImage BlogImage { get; set; }
}

public class BlogImage
{
    public int BlogImageId { get; set; }
    public byte[] Image { get; set; }
    public string Caption { get; set; }

    public int BlogForeignKey { get; set; }
    public Blog Blog { get; set; }
}

Answer 2

The above answer is absolutely correct.

Just for readers information: This has been explained nicely in official documentation

One-to-one

One to one relationships have a reference navigation property on both sides. They follow the same conventions as one-to-many relationships, but a unique index is introduced on the foreign key property to ensure only one dependent is related to each principal.

public class Blog
{
    public int BlogId { get; set; }
    public string Url { get; set; }

    public BlogImage BlogImage { get; set; }
}

public class BlogImage
{
    public int BlogImageId { get; set; }
    public byte[] Image { get; set; }
    public string Caption { get; set; }

    public int BlogId { get; set; }
    public Blog Blog { get; set; }
}

Note

EF will choose one of the entities to be the dependent based on its ability to detect a foreign key property. If the wrong entity is chosen as the dependent, you can use the Fluent API to correct this.