Equivalent for .HasOptional in Entity Framework Core 1 (EF7) : <p>Consider two classes.</p>

<pre><code>public class File
{
    [Key]
    public string Id { get; set; }

    public string Message_Id { get; set; }

    internal Message Message { get; set; }
}

public class Message 
{
    [Key]
    public string Id { get; set; }       
}
</code></pre>

<p>In EF6, for N : 1..0 relation there was this fluent API.</p>

<pre><code>modelBuilder.Entity&lt;File&gt;()
            .HasOptional(e =&gt; e.Message ).WithMany().HasForeignKey(e =&gt; e.Message_Id);
</code></pre>

<p>What is equivalent in Entiity Framework Core 1?</p>

<p>Thank you</p>
