Entity Framework foreign key error with delete
<p>{"The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted."}</p>

<pre><code>public partial class child
    {
        public int  childID { get; set; }
        public int fatherID { get; set; }


        public virtual father CorrelationCorrecrionGroup { get; set; }

public class father : EntityTypeConfiguration&lt;child&gt;
    {
        public father ()
        {
            // Primary Key
            this.HasKey(t =&gt; t.childID);

            //code...



             //Relationships    
            this.HasRequired(t =&gt; t.fhther)
                .WithMany(t =&gt; t.child);
</code></pre>

<p>I'm deleting a child item and get an error.
Thank you.</p>