parent-child relation with two children...

I have something like:
entity (id, live_version_id NULL, preview_version_id NULL)
version (id, parent_entity_id)

where version.parent_entity_id must be a valid entity.id, and that entity must have the version.id in either live/preview version_id. Is this a common set up? Does it make sense? Is there a way to express this requirement at the DB level (I'm specifically using MySQL, but in general I'd like to know)

Or, did I model my data in a bad way?

Thanks,
Daniel.
--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>
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