Prefetch match composite namevar(s)

filemapper.rb

    def prefetch(resources = {})
      # generate hash of {provider_name => provider}
      providers = instances.inject({}) do |hash, instance|
        hash[instance.name] = instance
        hash
      end

      # Identify the namevar(s) for the type
      nv_properties = resource_type.properties.select(&:isnamevar?).map(&:name)
      nv_params = resource_type.parameters.select do |param|
        resource_type.paramclass(param).isnamevar?
      end
      namevars = (nv_properties | nv_params || [:name])

      # For each prefetched resource, try to match it to a provider
      resources.each_pair do |resource_name, resource|
        if provider = providers[resource_name]
          resource.provider = provider
        end
      end
    end

Checkout resource_type.key_attribute_parameters or resource_type.class.key_attribute_parameters - can't remember which, but that will give you the namevars.