Comment 9 for bug 1471316

Sorry for late.

As long as I see comment #6 and #7, it seems we can safely return None for subnetpool_id when a cidr is specified and a subnet_pool is not specified.

IMO an important thing in the API perspective is that the defined attributes in a specific API version (or extension at the moment) should be visible in a response. Some attribute can be optional in a POST request, but we have a consistent logic to determine the default value for each case and we can return a deterministic value in a response.
Another important point is that we return a consistent response for a POST/PUT/GET request.
We can satisfy the above my points by returning None for subnetpool_id when only CIDR is specified.