MMBase

Like it is being used in streamsourcescaches.id field in the Streams application:

    <datatype base="nodenumber" xmlns="http://www.mmbase.org/xmlns/datatypes">
      <required value="true" enforce="never" />
        <enumeration enforce="never"> <!-- enforcing is very bad for performance -->
        <query type="mediasources" xmlns="http://www.mmbase.org/xmlns/searchquery" />
      </enumeration>
    </datatype>