The 2.0 DTD does not support to have an inverse attribute on an idbag.

<!ELEMENT idbag (
meta*,
(cache|jcs-cache)?,
collection-id,
key,
(element|many-to-many|composite-element|many-to-any)
)>
<!ATTLIST idbag name CDATA #REQUIRED>
<!ATTLIST idbag access CDATA #IMPLIED>
<!ATTLIST idbag table CDATA #IMPLIED> <!-- default: name -->
<!ATTLIST idbag schema CDATA #IMPLIED> <!-- default: none -->
<!ATTLIST idbag lazy (true|false) "false">
<!ATTLIST idbag cascade (none|all|save-update|delete|all-delete-orphan|delete-orphan) #IMPLIED> <!-- default: none -->
<!ATTLIST idbag order-by CDATA #IMPLIED> <!-- default: none -->
<!ATTLIST idbag where CDATA #IMPLIED> <!-- default: none -->
<!ATTLIST idbag batch-size CDATA "1">
<!ATTLIST idbag outer-join (true|false|auto) "auto">
<!ATTLIST idbag check CDATA #IMPLIED> <!-- default: none -->

Is it really unsupported? If yes, why is that so?

It doesn't make sense for an <idbag>. Just use a <bag> instead :)

Why? I have a table with the following format:

ID
INX_TO
INX_FROM
ASSOCIATION_TYPE

To regroup some reflexive associations. There are different types of associations (ASSOCIATION_TYPE). An idbag is nice because I can remove or add associations without always removing all the list.

To mapping look like this:

<idbag name="duplicateDaughters" table="inx_reflexive" where="association_type='DUPLICATED'" lazy="true">
<collection-id column="ID" type="long">
<generator class="sequence">
<param name="sequence">INX_REFLEXIVE_DB_ID</param>
</generator>
</collection-id>
<key column="inx_from"/>
<composite-element class="InxReflexive">
<property name="duplicated" column="association_type"/>
<many-to-one name="inxTo" class="Inx" column="inx_to"/>
</composite-element>
</idbag>

So we are using a composite element to map the association type. In this case, I also have a duplicateMother mapping. So duplicateDauthers should be the inverse of duplicateMother (which is in fact a one-to-many).

Where doesn't it make sense?