code: public void put(RowKey key, Tuple value) {
  		// instead of setting it to null, core must use remove
  		Contracts.assertNotNull( value, "association.put value" );
  		currentState.put( key, new AssociationOperation( key, value, PUT )  );
  	}
docs: Adds the given row to this association, using the given row key.
  The row must not be null, use the {@link org.hibernate.ogm.model.spi.Association#remove(org.hibernate.ogm.model.key.spi.RowKey)}
  operation instead.
  
  @param key the key to store the row under
  @param value the association row to store