docs: def group_by(resources, key):
      """Return a mapping of key value to resources with the corresponding value.
  
      Key may be specified as dotted form for nested dictionary lookup
      """
      resource_map = {}
      parts = key.split('.')
      for r in resources:
          v = r
          for k in parts:
              v = v.get(k)
              if not isinstance(v, dict):
                  break
          resource_map.setdefault(v, []).append(r)
      return resource_map