def serialize_association(ass):
    """
    Serialize an xtuml metamodel association.
    """
    s1 = '%s %s (%s)' % (ass.source_link.cardinality, ass.source_link.
        to_metaclass.kind, ', '.join(ass.source_keys))
    if ass.target_link.phrase:
        s1 += " PHRASE '%s'" % ass.target_link.phrase
    s2 = '%s %s (%s)' % (ass.target_link.cardinality, ass.target_link.
        to_metaclass.kind, ', '.join(ass.target_keys))
    if ass.source_link.phrase:
        s2 += " PHRASE '%s'" % ass.source_link.phrase
    return 'CREATE ROP REF_ID %s FROM %s TO %s;\n' % (ass.rel_id, s1, s2)
