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Entity Beans and Composite-keys

JeanLouis Marechaux
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Joined: Nov 12, 2001
Posts: 906
Hi,
With Antity beans, I have (not really, but I prefer) to declare a primary Key class, lets say CustomerPK.
It's quite simple if my primary key is only 1 value, let's say custID.
But if it is a composite key, what do you think about it.
I can still declare all the field needed to compose the key, but I wonder if I will have other problems (maybe with hashcode and equals methods, or performance probel etc..)

If you guys already made an intensive use of composite keys with CMP, please share your experience.
TIA


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Kyle Brown
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Joined: Aug 10, 2001
Posts: 3892
    
    5
Composite keys are fine if that's the natural primary key of the table. This happens fairly often in bottom-up or meet-in-the-middle mappings.
And yes, you have to make your equals method check both keys, but that's generally not hard. Likewise, combine the hash of the two parts of your key to make up the hashCode. There shouldn't be any problems.
Kyle


Kyle Brown, Author of Persistence in the Enterprise and Enterprise Java Programming with IBM Websphere, 2nd Edition
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Tim Holloway
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Joined: Jun 25, 2001
Posts: 16065
    
  21

Sometimes you have no choice. A project I worked on last year had over half the datbase tables being referenced with no single column that would define a row as unique.
There are 3 methods in the Primary Key class that deserve attention when using multi-part keys:
1. The hashCode() method. You CAN be sloppy with this, but ideally you'll be tuning hashCode() to minimize collisions and that's true no matter how many parts make up the key.
2. The equals() method. If you don't compare on ALL fields in the key, you won't REALLY know if the comparand is or is not referring to the same EJB as the comparator.
3. The toString() method. Some EJB containers (and/or your debugging logic) like you to implement this method. It's useful if when displays something that helps you pin down which DBMS row you're looking at. I generally just "toString()" each key subcomponent and glue them together with slashes, dashes, or something else that makes it obvious what each component's individual value is.


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subject: Entity Beans and Composite-keys