code: function Id() {
      return function (targetObject, propertyKey) {
          var entityMetadata = targetObject;
          if (entityMetadata.idProperty) {
              throw "Cannot set primary key to '" + propertyKey + "', it is already set to '" + entityMetadata.idProperty + "'";
          }
          entityMetadata.idProperty = propertyKey;
      };
  }
docs: Created by Papa on 8/20/2016.
   
  Annotates Id fields of Entities.
  
  @returns {function(any, string)}
  @constructor