[hibernate-issues] [Hibernate-JIRA] Created: (HHH-5637) Allow overridden column names with "table per concrete class" inheritance

[Mark A. Ziesemer (JIRA)]

Allow overridden column names with "table per concrete class" inheritance
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                 Key: HHH-5637
                 URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-5637
             Project: Hibernate Core
          Issue Type: Improvement
          Components: core
    Affects Versions: 3.5.6
            Reporter: Mark A. Ziesemer


Per http://docs.jboss.org/hibernate/stable/core/reference/en/html/inheritance.html#inheritance-tableperconcrete for "Table per concrete class":
{quote}
The limitation of this approach is that if a property is mapped on the superclass, the column name must be the same on all subclass tables.
{quote}

At one point, this documentation also mentioned ([source|http://viewvc.jboss.org/cgi-bin/viewvc.cgi/hibernate/core/trunk/documentation/manual/src/main/docbook/en-US/content/inheritance_mapping.xml?r1=14652&r2=15831]):
{quote}
(We might relax this in a future release of Hibernate.)
{quote}

I've not found a related task for this, so I'm creating this task as a request to support this.

For one use case, I have several related tables that would work very well with polymorphic queries.  They have many related columns, but with slightly different column names, e.g. "{{header_id}}" vs. "{{item_header_id}}" (this is arguably poor DB schema design, but is a 3rd party DB that we can't modify).  I'm currently able to work with it by using the "implicit polymorphism" alternative.  In some cases, I'm using {{@AttributeOverride}}, and in other cases the properties are defined in the parent abstract class as {{@Transient}}, and then overridden in the child concrete classes with the appropriate {{@Column}} annotated per-class.

Unfortunately, without using {{TABLE_PER_CLASS}}, SQL unions are not used.  This negatively impacts database performance, and doesn't allow for DB-side ordering and setting a maximum # of results.

When attempting {{TABLE_PER_CLASS}}, Hibernate already generates SQL similar to the following:
{code:SQL}
  Select entityId_, field1_, field2_ From (
    Select entityId As entityId_, field1 As field1_, field2 As field2_ From table1
    Union Select entityId As entityId_, field1 As field1_, field2 As field2_ From table2
  );
{code}

Hibernate should allow {{@AttributeOverride}}'s or {{@Column}} annotations on the subclasses to override the column names per table.  For example:
{code:SQL}
  Select entityId_, field1_, field2_ From (
    Select header_id As entityId_, field1 As field1_, field2 As field2_ From table1
    Union Select item_header_id As entityId_, field1 As field1_, field2 As field2_ From table2
  );
{code}

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