code: function mergeLinked(source, target, pathArr, ancestorType) {
      var linked, type, res;
  
      if (pathArr.length) {
        // the external is referenced by the ancestor
        if (source.links && source.linked && source.links[ancestorType + "." + pathArr.join(".")]) {
          type = source.links[ancestorType + "." + pathArr.join(".")].type;
          linked = source.linked[type];
        }
      } else {
        // ancestor is the sought external
        type = ancestorType;
        linked = source[ancestorType] || [];
      }
  
      target.linked[type] = _.isArray(res = target.linked[type]) ? _.uniq(res.concat(linked), function(i){ return i.id}) : linked;
    }
docs: Replaces the target linked property with the external data.
  
  @param {Object} The source from the external lookup.
  @param {Object} The target resource.
  @param {Object} The include properties.
  @param {Object} The lookup resource type.
  @api private