Databases
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    <p>I'd suggest you first get the Java-only OO model right and then worry about which Hibernate annotations you use. If you find you need to <em>legitimately</em> alter your model for Hibernate to fit in, you can always do so. </p> <p>In the end, this type of problem is typically solved with some variation of the <a href="http://en.wikipedia.org/wiki/Composite_pattern" rel="nofollow">Composite pattern</a> (don't focus too much on the pattern thing, just on the structure and idea behind it.)</p> <p>Using relational database lingo (in a rather relaxed manner): </p> <p>A. If your domains (root domains and sub-domains) are <em>relations</em> (collections of n-tuples in a table with no duplicates and with a discernible primary key) <strong>and</strong></p> <p>B. your domains and sub-domains <em>have similar structure</em>, <strong>then</strong></p> <p>C. You might be able to store all of them in the same physical table by defining a <strong>"parent"</strong> foreign key such that the parent FK of one tuple maps to the primary key of another one.</p> <p>Most importantly, this recursive relationship must be acyclic. How you go about it structurally is up to your problem domain (do you have one root domain, or you can have multiple, unrelated root domains?) A root domain can be denotated by having either a NULL parent foreign key, or with a condition where a root domain tuple's parent foreign key equals its own primary key. Either one has pros and cons (which are the subject of typically stupid flame wars.)</p>
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    1. POHow to model a tree with Hibernate?
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    1. COI strongly disagree with the usage of Composite pattern (see my answer for more details). But if you solved that problem with "some variation" then you should explain that variation in some details.
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    2. COThe variation I spoke of is by having all leafs and composites be of the same concrete type (see Thomas' answers for details.) Of course a strict (and thus naive) interpretation of what "composite pattern" means will result in many problems (independent of hibernate being in the picture.) And the reality is that all "kosher", by-the-book implementation of patterns will also run into problems. The name "composite" should not elicit visions of strictly different leaf and composite types but instead of *things* that can also be composed of other, possibly type-equivalent *things*.
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