[Russland]

1. Override filter() method on ExtCodeValue field (ExtCodeValueTable data source) like this:

public void filter(str _filterStr, NoYes _clearPrev)
{
    if(!_filterStr)
        _filterStr = InventTable::findRecId(ExtCodeValueTable.ExtCodeRelationRecId).ItemId;

    InventTable_ds.query().dataSourceTable(tableNum(ExtCodeValueTable)).joinMode(JoinMode::InnerJoin);
    InventTable_ds.executeQuery();

    super(_filterStr, _clearPrev);
}

2. Override removeFilter() method on InventTable data source:

public void removeFilter()
{
    InventTable_ds.query().dataSourceTable(tableNum(ExtCodeValueTable)).joinMode(JoinMode::OuterJoin);
    InventTable_ds.executeQuery();

    super();
}

Hope it will solve your problem