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copied!<blockquote> <p>Imagine 2 tables in a relational database, e.g. Person and Billing. There is a (non-mandatory) OneToOne association defined between these entities,</p> </blockquote> <p>Lazy fetching is conceptually not possible for non-mandatory OneToOne by default, Hibernate has to hit the database to know if the association is <code>null</code> or not. More details from this old wiki page:</p> <blockquote> <h3><a href="https://community.jboss.org/wiki/Someexplanationsonlazyloadingone-to-one" rel="nofollow noreferrer">Some explanations on lazy loading (one-to-one)</a></h3> <p>[...]</p> <p>Now consider our class B has one-to-one association to C</p> <pre><code>class B { private C cee; public C getCee() { return cee; } public void setCee(C cee) { this.cee = cee; } } class C { // Not important really } </code></pre> <p>Right after loading B, you may call <code>getCee()</code> to obtain C. But look, <code>getCee()</code> is a method of YOUR class and Hibernate has no control over it. Hibernate does not know when someone is going to call <code>getCee()</code>. That means Hibernate must put an appropriate value into "<code>cee</code>" property at the moment it loads B from database. If proxy is enabled for <code>C</code>, Hibernate can put a C-proxy object which is not loaded yet, but will be loaded when someone uses it. This gives lazy loading for <code>one-to-one</code>. </p> <p>But now imagine your <code>B</code> object may or may not have associated <code>C</code> (<code>constrained="false"</code>). What should <code>getCee()</code> return when specific <code>B</code> does not have <code>C</code>? Null. But remember, Hibernate must set correct value of "cee" at the moment it set <code>B</code> (because it does no know when someone will call <code>getCee()</code>). Proxy does not help here because proxy itself in already non-null object.</p> <p>So the resume: <strong>if your B->C mapping is mandatory (<code>constrained=true</code>), Hibernate will use proxy for C resulting in lazy initialization. But if you allow B without C, Hibernate just HAS TO check presence of C at the moment it loads B. But a SELECT to check presence is just inefficient because the same SELECT may not just check presence, but load entire object. So lazy loading goes away</strong>.</p> </blockquote> <p>So, not possible... by default. </p> <blockquote> <p>Is there a workaround for this potential performance disaster (other than not using a shared primary key at all)? Thank you for all your ideas.</p> </blockquote> <p>The problem is not the shared primary key, with or without shared primary key, you'll get it, the problem is the <strong>nullable</strong> OneToOne.</p> <p><strong>First option</strong>: use bytecode instrumentation (see references to the documentation below) and <em>no-proxy</em> fetching:</p> <pre><code>@OneToOne( fetch = FetchType.LAZY ) @org.hibernate.annotations.LazyToOne(org.hibernate.annotations.LazyToOneOption.NO_PROXY) </code></pre> <p><strong>Second option</strong>: Use a fake <code>ManyToOne(fetch=FetchType.LAZY)</code>. That's probably the most simple solution (and to my knowledge, the recommended one). But I didn't test this with a shared PK though.</p> <p><strong>Third option</strong>: Eager load the Billing using a <code>join fetch</code>.</p> <h3>Related question</h3> <ul> <li><a href="https://stackoverflow.com/questions/1444227/making-a-onetoone-relation-lazy">Making a OneToOne-relation lazy</a> </li> </ul> <h3>References</h3> <ul> <li>Hibernate Reference Guide <ul> <li><a href="http://docs.jboss.org/hibernate/core/3.3/reference/en/html/performance.html#performance-fetching-proxies" rel="nofollow noreferrer">19.1.3. Single-ended association proxies</a></li> <li><a href="http://docs.jboss.org/hibernate/core/3.3/reference/en/html/performance.html#performance-fetching-lazyproperties" rel="nofollow noreferrer">19.1.7. Using lazy property fetching</a> </li> </ul></li> <li>Old Hibernate FAQ <ul> <li><a href="http://web.archive.org/web/20071021233649/http://www.hibernate.org/117.html#A18" rel="nofollow noreferrer">How do I set up a 1-to-1 relationship as lazy?</a></li> </ul></li> <li>Hibernate Wiki <ul> <li><a href="https://community.jboss.org/wiki/Someexplanationsonlazyloadingone-to-one" rel="nofollow noreferrer">Some explanations on lazy loading (one-to-one)</a></li> </ul></li> </ul>

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