JPA - Optional columns

Question

We have many customers data in separate databases per customer which should have the same schema/table structures. However there is a table that has extra columns in some databases compared to others.

For example for customer A there is a table X with columns a, b, c, d. For customer B there is a table X with columns a, c, d. I need to capture b if it exists but can ignore it if not.

Is there a way to tell JPA to ignore those columns if they don't exist? @Basic(optional=true) reads exactly like what I want but the documentation indicates it is for another purpose.

Currently I get, as expected, Unknown column 'table.field' in 'field list'

P.S. I can't just add the columns to the databases that don't have them unfortunately.

Answer 1

@Basic(optional=true) it's just to tell the schema generator (if any) that the field can hold null values, not that the field might or might not be present.

A possible solution for your problem that comes to my mind is to use a class hierarchy, defining a common parent class with @MappedSuperclass instead of @Entity and then defining each concrete class for each database extending from that one.

With @MappedSuperclass the JPA implementation wouldn't look for a table to match those fields so you might even have some empty entity classes (extending the super class) just to define your model.