This is the code I ended up with (from the QSqlRelationalDelegate 's code), and works!
Any comments are welcome.
Qt Code: 
  1. int positionColumn = 2;
  2.     qDebug()<< "BookDelegate.createEditor index.column()=" << (index.column()) << " option=" << option ;
  3.     if (index.column() == positionColumn){
  4.         const QSqlRelationalTableModel *sqlModel = qobject_cast<const QSqlRelationalTableModel *>(index.model());
  5.         QSqlTableModel *childModel = sqlModel ? sqlModel->relationModel(index.column()) : nullptr;
  6.         if (!childModel)
  7.             return QStyledItemDelegate::createEditor(parent, option, index);
  8.  
  9.         QComboBox *combo = new QComboBox(parent);
  10.         combo->setModel(childModel);
  11.         combo->setModelColumn(1);
  12.         combo->installEventFilter(const_cast<BookDelegate *>(this));
  13.         childModel->select();
  14.         return combo;
  15.     }
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