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    <p>A few problems with your mapping... You have a bidirectional relationship and NHibernate needs to know which way to update it. In the OO world, references only go one way and there is no way for NHibernate to know that Parent->Children is the same FK as Child->Parent. Right now you have Child->Parent set to ReadOnly(). This is telling NHibernate not to update this property. So it tries to insert the Child (with a null parent) and then update the FK from the Parent side. This isn't working if you have a not null constraint on your FK. The usual way to map this is to use Inverse=true on the parent side and let the child worry about persistence. (It's your job in the OO model to ensure that the Parent->Children collection contains the same set of references as the set of Child->Parent relationships.)</p> <pre><code>public class ParentMap : ClassMap&lt;Parent&gt; { public ParentMap() { Id(x =&gt; x.ParentId).GeneratedBy.Native(); Map(x =&gt; x.Name); HasMany(x =&gt; x.Children).KeyColumn("ParentId").Inverse().Cascade.SaveUpdate(); } } public class ChildMap : ClassMap&lt;Child&gt; { public ChildMap() { Id(x =&gt; x.ChildId).GeneratedBy.Native(); Map(x =&gt; x.Name); References(x =&gt; x.Parent).Column("ParentId").Not.Nullable(); // Removed ReadOnly() } } </code></pre> <p>The SQL statements sent to the database when saving are now:</p> <pre><code>INSERT INTO [Parent] (Name) VALUES ('P1' /* @p0 */) select SCOPE_IDENTITY() INSERT INTO [Child] (Name, ParentId) VALUES ('C1' /* @p0 */, 1 /* @p1 */) select SCOPE_IDENTITY() </code></pre>
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    1. USJames Kovacs
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    1. COJames, okay removing the ReadOnly() qualifier worked to get the test passing. However, now I am confused about the meaning of ReadOnly(). My understanding was that ReadOnly() means that NHibernate will never update the column after the initial insert, not that it will never even assign a value upon insert.
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      1. USKenneth Baltrinic
    2. COReadOnly() means that the value is never written either on insert or update. It is primarily used for lookup tables and calculated columns.
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    3. COHmm, I knew that, now that you mention it. I think I confused ReadOnly() as a property attribute with ReadOnly as an entity attribute. Must be getting old. :) Thanks for the help!
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