Use multiple columns for Discriminator in Inheritance Tree with single-table


Kraen David Christensen
 

We have a table like: node(nodetype, subnodetype, ...)
From this one table we want to have several persistent java classes.
Both node.nodetype and node.subnodetype is needed to discriminate the java class we need (sometimes only nodetype is enough).
As far as I can see JDO/JPA only allows ONE discriminator column.
Is there anyway I can customize and build my own discriminator type (eg. by specifying something like:
@Discriminator(customStrategy = "MyStrategy")
on node table?
I've tried with no luck - but perhaps this is not possible.
It might be that I should write my own StoreManager extension for this?
And would that be recommended/easy?

Best regards, Kr├Žn David Christensen


Andy
 
Edited

Yes, JDO, JPA (and Jakarta Persistence) all allow 1 discriminator (value) only. JDO metadata strictly speaking permits multiple columns for some reason, but there is only space for 1 value and that's what DataNucleus has always stuck to. Multiple discriminator values makes little sense to me FWIW.

I don't see any way of having multiple discriminator columns by using extensions with current DataNucleus code.

Options
  • Update DN code to cater for multiple columns (and values) for each class, and then provide a mechanism for getting class name from the column values. This is potentially a lot of work, and would have to retain compatibility with standard JDO / JPA handling.
  • Update your DB to only have a single (surrogate) column and use standard JDO discriminator handling.
  • Update your class(es) to map on to a single one of the (surrogate) columns and optionally read in the other column (if it is still important) into a field of the class(es).