Persistent collections are a feature!

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In the past few months I got the impression that something in our explanation of object/relational mapping has gone a little wrong. I'm still not sure that this is the fault of any documentation or presentation slide, it's just something that naturally happened...

It seems to me that many many users of Hibernate (and very likely other similar tools) think that they absolutely have to map a collection. Basically, many developers, especially if they have not been exposed to some data management basics and have survived with a purely object-oriented view of the world, seem to think that persistent collections are an absolute must-have. Sometimes it's quite a surprise when I explain how you can write a sophisticated and complete Hibernate application without ever mapping a single collection!

This is even more important if you are only starting to learn Hibernate; mapping a collection right is definitely the biggest step in the learning curve. I want to clear up some of the confusion around collection mappings (the actual reason is a guy who wants us to declare unusable on the Hibernate Annotations website because the current beta doesn't allow you to map a Map of entities across a join table...)

Every time you think you have to map a collection you have to ask yourself /Why/. A mapped persistent collection is a /feature/, not a requirement. What does this feature give you:

  • You no longer have to retrieve the data you are looking for with a query, you can just iterate through a collection. Naturally, this iteration can always, no exception, be replaced with a query.
  • You no longer have to maintain a relationship (please, stop calling it relation) between two pieces of data manually. For example, you can just put a child object into a collection of a parent object (I really hate this explanation but it's a convenient lie) and have it saved automatically. Naturally, this cascading persistence can always, no exception, be replaced with an explicit call of the persistence manager.
  • An object/relational mapping software can make optimizations when you access the collection, such as batch fetching or /extra/ lazy behavior if you just call size() on it. These are goodies that might make your life easier, but they can always, no exception, be replaced with a query. (Actually, this is conceptually true, but doesn't work for batch and subselect collection fetching in Hibernate 3.1 - addition of these features to the query API is on the list.)

So, you map a collection if you want one of these features. You don't map it if these features are of no advantage to you! In other words: a relational database has foreign keys (copies of keys and constraints to hold it together) and nothing else. All foreign key relationships can be represented in Java with many-to-one associations. Everything else is sugar, features, convenience. Yes, you often want this convenience and you are right to expect it from an object/relational mapping software. But you don't have to force a collection mapping, especially if you don't know how to do it right or what you really get from it.

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