code: @Override
  	public String getSingleRowKeyColumnNotContainedInAssociationKey() {
  		String nonKeyColumn = null;
  
  		for ( String column : getRowKeyColumnNames() ) {
  			if ( !isKeyColumn( column ) ) {
  				if ( nonKeyColumn != null ) {
  					// more than one column which is not contained in the association key; thus rows of this
  					// association will be represented by DBObjects rather than single String, int etc. objects
  					return null;
  				}
  				else {
  					nonKeyColumn = column;
  				}
  			}
  		}
  
  		return nonKeyColumn;
  	}
docs: Returns the name of the single row key column which is not a column of this key itself, in case such a column
  exists.
  <p>
  If e.g. an association key contains the column "bankAccounts_id" and the row key columns are "bankAccounts_id"
  and "owners_id", this method will return "owners_id". But if the row columns were "bankAccounts_id", "owners_id"
  and "order", {@code null} would be returned as there were more than one column which are not part of the
  association key.
  
  @return the name of the single row key column which is not a column of this key itself or {@code null} if there
  is either no or more than one such column.