This worked in 1.10

curl -u user:password -H'Content-Type: application/json'  -X PUT -d'{"name": "a123", "title": "a123", "environment_id": 1}' http://localhost:3000/api/v2/hostgroups/5

and it works in Foreman as of 42ba771cc8bfc09257c15c5d388542d4d1b69358 if I leave out the environment_id parameter but not with it.

I've tried different parameters (environment_id, realm_id) but changing these fail with

{
         "error": {"message":"Resource hostgroup not found by id '5'"}
     }

The log has

08:03:43 rails.1   | 2016-09-01T08:03:43 0a08d560 [app] [I] Started PUT "/api/v2/hostgroups/5" for 127.0.0.1 at 2016-09-01 08:03:43 +0200
    08:03:43 rails.1   | 2016-09-01T08:03:43 0a08d560 [app] [I] Processing by Api::V2::HostgroupsController#update as JSON
    08:03:43 rails.1   | 2016-09-01T08:03:43 0a08d560 [app] [I]   Parameters: {"name"=>"a123", "title"=>"a123", "environment_id"=>1, "apiv"=>"v2", "id"=>"5", "hostgroup"=>{"name"=>"a123", "environment_id"=>1, "title"=>"a123"}}
    08:03:43 rails.1   | 2016-09-01T08:03:43 0a08d560 [app] [D] Setting current user thread-local variable to foreman_admin
    08:03:43 rails.1   | 2016-09-01T08:03:43 0a08d560 [app] [D] Setting current user thread-local variable to nil
    08:03:43 rails.1   | 2016-09-01T08:03:43 0a08d560 [app] [D] Setting current user thread-local variable to foreman_admin
    08:03:43 rails.1   | 2016-09-01T08:03:43 0a08d560 [app] [D] Setting current user thread-local variable to nil
    08:03:43 rails.1   | 2016-09-01T08:03:43 0a08d560 [app] [D] Setting current user thread-local variable to user
    08:03:43 rails.1   | 2016-09-01T08:03:43 0a08d560 [app] [I] Authorized user user(user)
    08:03:43 rails.1   | 2016-09-01T08:03:43 0a08d560 [app] [D] Setting current user thread-local variable to user
    08:03:44 rails.1   | 2016-09-01T08:03:44 0a08d560 [app] [I] Couldn't find Hostgroup with 'id'=5 [WHERE ("environments"."id" = 1 OR "environments"."name" = '1')] (ActiveRecord::RecordNotFound)
    08:03:44 rails.1   | 2016-09-01T08:03:44 0a08d560 [app] [I]   Rendered api/v2/errors/not_found.json.rabl within api/v2/layouts/error_layout (0.6ms)
    08:03:44 rails.1   | 2016-09-01T08:03:44 0a08d560 [app] [I] Completed 404 Not Found in 69ms (Views: 3.7ms | ActiveRecord: 47.3ms)