mapping1.yang

  module mappings {

    //The yang version - today only 1 version exists. If omitted defaults to 1.
    yang-version 1; 

    //a unique namespace for this toaster module, to uniquely identify it from other modules that may have the same name.
    namespace
      "http://netconfcentral.org/ns/fw"; 

    //a shorter prefix that represents the namespace for references used below
    prefix mappingservice;

    //Defines the organization which defined / owns this .yang file.
    organization "Inocybe Technologies";

    //defines the primary contact of this yang file.
    contact
      "Icaro Camelo <icamelo@inocybe.com>";

    //provides a description of this .yang file.
    description
      "YANG version of the Mapping Service.";

    //defines the dates of revisions for this yang file
    revision "2015-11-05" {
      description
        "Mapping module.";
    }
    
      grouping map {
      list outer-map {
          key "id";
          leaf id {
            type uuid;
          }
          list inner-map {
              key "inner-key";
              leaf inner-key {
                  type string;
              }
              leaf value {
                   type somedata;
              }
          }
      }

  } 
  container mappings {
       uses map;
   }
}

understood, but you most probably are going to use the inner-map as a list, because you don't know the key anyways.