code: public org.grails.datastore.mapping.query.api.ProjectionList distinct(String propertyName, String alias) {
          final Projection proj = Projections.distinct(Projections.property(calculatePropertyName(propertyName)));
          addProjectionToList(proj,alias);
          return this;
      }
docs: A projection that selects a distince property name
  @param propertyName The property name
  @param alias The alias to use