code: public static boolean organizeAssociationMapByRowKey(
  			org.hibernate.ogm.model.spi.Association association,
  			AssociationKey key,
  			AssociationContext associationContext) {
  
  		if ( association.isEmpty() ) {
  			return false;
  		}
  
  		if ( key.getMetadata().getRowKeyIndexColumnNames().length != 1 ) {
  			return false;
  		}
  
  		Object valueOfFirstRow = association.get( association.getKeys().iterator().next() )
  				.get( key.getMetadata().getRowKeyIndexColumnNames()[0] );
  
  		if ( !( valueOfFirstRow instanceof String ) ) {
  			return false;
  		}
  
  		// The list style may be explicitly enforced for compatibility reasons
  		return getMapStorage( associationContext ) == MapStorageType.BY_KEY;
  	}
docs: Whether the rows of the given association should be stored in a hash using the single row key column as key or
  not.