Redmine

btw, the previous line in the log is:

Processing IssueRelationsController#create to xml (for 192.168.0.11 at 2011-07-21 23:29:41) [POST]
Parameters: {"format"=>"xml", "issue_id"=>"49", "action"=>"create", "issue_to_id"=>"50", "relation_type"=>"precedes", "controller"=>"issue_relations"}
Filter chain halted as [:check_if_login_required] rendered_or_redirected.
Completed in 9ms (View: 0, DB: 5) | 401 Unauthorized [http://192.168.0.30/issues/49/relations.xml?issue_to_id=50&relation_type=precedes]

which is weird because I'm providing proper user name and password, it works for other requests.

I moved the parameters into POST params, but it didn't help:

Processing IssueRelationsController#create to xml (for 192.168.0.11 at 2011-07-21 23:43:10) [POST]
Parameters: {"format"=>"xml", "issue_id"=>"49", "action"=>"create", "controller"=>"issue_relations"}
Filter chain halted as [:check_if_login_required] rendered_or_redirected.
Completed in 9ms (View: 0, DB: 5) | 401 Unauthorized [http://192.168.0.30/issues/49/relations.xml?]

Processing IssueRelationsController#create to xml (for 192.168.0.11 at 2011-07-21 23:43:10) [POST]
Parameters: {"format"=>"xml", "issue_id"=>"49", "action"=>"create", "controller"=>"issue_relations"}
Completed in 62ms (View: 2, DB: 41) | 422 Unprocessable Entity [http://192.168.0.30/issues/49/relations.xml?]

Please, reread the doc. You need a relation parameter, which is a hash of the relation attributes (similar to the API for other objects).

ok, I see - need to provide the XML block as the request body:
<relation><issue_to_id>50</issue_to_id><relation_type>precedes</relation_type></relation>

this works