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Object data can be directly stored in a relational database, but the schema design is a greater issue. If the intention is that schema is a derivative of the object model - i.e. the "classical" approach to mapping (typical strategies) then the schema design may not be trivial. This kind of schema design is what many of the O/R tools try to automate and/or hide. However, other strategies are available, including one very interesting one which is possible due to the paths in openEHR data.
See this wiki page for an approach called 'node+path' which shows how a relational database could be used to store path-based archetyped data such as that found in openEHR.