unique keys on inherited classes

hi folks,

in many of my applications i'm deriving all classes from the base 
_inode, which stores general information needed for mirroring.
the _inode class defines an inode_id (cluster-wide object id)
and an timestamp field for the modification time.

I've defined an unique index on the inode_id field of _inode.
This doesn't seem to be inherited to the derived classes.

The goal is to prevent double inode_id values in all _all_ 
derived classes. How could i achieve this ?

cu
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