code: public Metamodel getMetamodel(String persistenceUnit)
      {
          Map<String, Metamodel> model = getMetamodelMap();
          return persistenceUnit != null && model.containsKey(persistenceUnit) ? model.get(persistenceUnit) : null;
      }
docs: Gets the metamodel.
  
  @param persistenceUnit
  the persistence unit
  @return the metamodel