If it is object relation list (plural relation datatype), then you need to use extended attribute filter, there are at least a few on projects.ez.no. If it is object relation, then you should be able to use attribute filter with object id's for filter value(s).
thank you Andre, I have got the examples you mentioned from ez.no
{fetch( 'content',
'list',
hash( 'parent_node_id', 42,
'attribute_filter', array( array( 'article/image',
'=',
87 ) ) ) )} This example demonstrates how to do attribute filtering using an attribute of the "Object relation" datatype. Only article nodes that store relation to the specified content object (object ID = 87) in their "image" attribute will be included in the result.
I checked that one, but since the first line is talking about reverse relations I was assuming it wouldn't work for normal relations... but since this extended filter works using an attribute and object id as input, it should support my needs :)