After a night of sleep I got an idea myself. I just define two one way mappings and make one of them indexed. It also turns out indexing is defined the same way (after the property name) even if you declare a different get-method
or set-method
.
<mapping type="one-way">
<class-a>Foo</class-a>
<class-b>Bar</class-b>
<field>
<a>someString</a>
<b set-method="setSomeBarString">someBarString</b>
</field>
</mapping>
<mapping type="one-way">
<class-a>Bar</class-a>
<class-b>Foo</class-b>
<field>
<a get-method="getSomeBarString">someBarString[0]</a>
<b>someString</b>
</field>
</mapping>