Comment 7 for bug 1722842

Let me state the problem in an hopefully clearer way, based on an example that happens to be what I'm working on:
- the starting point is the BGPVPN API extension, that has introduced a bgpvpn' resource, with 'network_association' and 'router_association' sub-resources
- we are adding a BGPVPN-routes-control API extension, which:
  * introduces a new 'port_association' sub-resource (easy come, easy go, nothing more about this below)
  * introduces a new attribute for the 'bgpvpn' resources (easy come, easy go, nothing more about this below)
  * introduces a new attribute for the 'router_association' sub-resource: this is something, I think, the code in neutron.api.extensions does not currently support

(So the problem I bring up is not that it would be impossible to do a PUT on an attribute of a sub-resource)

Since, yesterday, I have been reading more code, and found that the 'qos-bw-limit-direction' extension actually does add an attribute to the 'bandwidth_limit_rules' sub-resource of the 'policy' resource (see [1]). Since this merged in pike and there are tempest API tests for it, I have to assume this works and that I got something wrong, but I don't know what yet.

[1] http://git.openstack.org/cgit/openstack/neutron/tree/neutron/extensions/qos_bw_limit_direction.py#n56