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copied!<p>EF Code First supports <code>1:1</code> and <code>1:0..1</code> relationships. The latter is what you are looking for ("one to zero-or-one").</p> <p>Your attempts at fluent are saying <strong>required on both ends</strong> in one case and <strong>optional on both ends</strong> in the other.</p> <p>What you need is <em>optional</em> on one end and <em>required</em> on the other.</p> <p>Here's an example from the Programming E.F. Code First book </p> <pre><code>modelBuilder.Entity<PersonPhoto>() .HasRequired(p => p.PhotoOf) .WithOptional(p => p.Photo); </code></pre> <p>The <code>PersonPhoto</code> entity has a navigation property called <code>PhotoOf</code> that points to a <code>Person</code> type. The <code>Person</code> type has a navigation property called <code>Photo</code> that points to the <code>PersonPhoto</code> type.</p> <p>In the two related classes, you use each type's <em>primary key</em>, not <em>foreign keys</em>. i.e., you won't use the <code>LoyaltyUserDetailId</code> or <code>PIIUserId</code> properties. Instead, the relationship depends on the <code>Id</code> fields of both types. </p> <p>If you are using the fluent API as above, you do not need to specify <code>LoyaltyUser.Id</code> as a foreign key, EF will figure it out. </p> <p>So without having your code to test myself (I hate doing this from my head)... I would translate this into your code as</p> <pre><code>public class PIIUser { public int Id { get; set; } public LoyaltyUserDetail LoyaltyUserDetail { get; set; } } public class LoyaltyUserDetail { public int Id { get; set; } public double? AvailablePoints { get; set; } public PIIUser PIIUser { get; set; } } protected override void OnModelCreating(DbModelBuilder modelBuilder) { modelBuilder.Entity<LoyaltyUserDetail>() .HasRequired(lu => lu.PIIUser ) .WithOptional(pi => pi.LoyaltyUserDetail ); } </code></pre> <p>That's saying LoyaltyUserDetails <code>PIIUser</code> property is <em>required</em> and PIIUser's <code>LoyaltyUserDetail</code> property is optional.</p> <p>You could start from the other end:</p> <pre><code>modelBuilder.Entity<PIIUser>() .HasOptional(pi => pi.LoyaltyUserDetail) .WithRequired(lu => lu.PIIUser); </code></pre> <p>which now says PIIUser's <code>LoyaltyUserDetail</code> property is optional and LoyaltyUser's <code>PIIUser</code> property is required.</p> <p>You always have to use the pattern HAS/WITH.</p> <p>HTH and FWIW, one to one (or one to zero/one) relationships are one of the most confusing relationships to configure in code first so you are not alone! :)</p>

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