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    <p>I see two problems with using one-to-one references:</p> <ol> <li>Parent has zero or one child1, and zero or one child2. My understanding is that you use one to one when it's ALWAYS one.</li> <li>Your IDs are getting really complicated. The idea with one-to-one is that the ID's are the same between the two entities, and you've got Parent with a two-part primary key, and Child with a different four part primary key.</li> </ol> <p>The other addendum to 2. is that if you're using 1-1, then the ID of parent and child should be the same. But if the parent has two children, they can't both have the same ID! So there's a real data modeling problem. Also, the fact that child type is part of the key is also a bad smell, cause I expect child type is part of your business logic, </p> <p>I think what you want is what Hibernate calls a 'unidirectional one-to-one association on a foreign key'. The <a href="http://docs.jboss.org/hibernate/orm/4.1/manual/en-US/html_single/#assoc-unidirectional-121" rel="nofollow">Hibernate ORM manual</a> shows how to do this in XML, here's an <a href="http://www.vaannila.com/hibernate/hibernate-example/hibernate-mapping-one-to-one-using-annotations-1.html" rel="nofollow">example</a> with Annotations, although without the unique property set.</p> <p>I'm going to assume there's no outside business reason why your primary keys have to be the way that they are, and suggest that you</p> <ol> <li>Change Parent and Child to each have to have a single field primary key, using @Id and @GeneratedValue(strategy=GenerationType.AUTO).</li> <li>Change Parent to have the references to child1 and child2 be @ManyToOne(unique=true)</li> <li>If you want, change Child as detailed in the Hibernate manual so that it has a reference to the Parent.</li> </ol> <p>You will lose the 'feature' that parents and children have the same ID, but gain massive simplicity in your object ID scheme, making database operations faster and the code simpler to read.</p> <p>If there is an outside reason that you haven't specified as to why Parent and/or Child need multi-part primary keys, this answer will need some modification.</p> <p>I also second the point that @barsju made about using inheritance, or even entirely independent classes, to do the two Child types. If you use ID's the way I lay out here, you can make Parent have references to the concrete subtypes, and the query will work fine.</p>
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    1. POTwo almost identical OneToOne in Entity, custom join annotation
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    1. COas I stated I only have `oid` and `id` as keys, because if I create another single field key for `Child` it will be lost. I select Parent from DB, transform it to XML (or DOM) -> now I don't have place where to put the `Child` PK...then I modify DOM somewhere and after that I transform XML (or DOM) to `Parent` and `Child` POJOs...and I'm doomed, no more PK in there...children will be always created as new. XML schema is fixed. I know that this PK stuff is madness :) As @barsju said - inheritance is probably solution. Didn't have time to test it, will do in next few days.
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    2. COOK, I didn't see any mention in the question about loading via Hibernate and then converting to XML. So these Java classes are some kind of intermediate representation? When you say you "don't have place...to put the Child PK", does that mean you don't have control over the XML representation, and can't add attributes to it?
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    3. COYes. These are only POJO classes to save/load XML representation from relational DB and I can't change XSD schema. I mentioned it in question - in I need to save XML.. part as well as in some other comments. But thanks for help anyway :)
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