Qt 4.8.4, Firebird 2.5.

I have code like this:
Qt Code: 
  1. // model - QSqlRelationalTableModel
  2.     if( model->database().transaction() )
  3.         {
  4.             int indexDialog;
  5.  
  6.             if( _index == 0 )
  7.             {
  8.                 model->insertRow( model->rowCount() );
  9.                 indexDialog = model->index( model->rowCount() - 1, 0 ).row();
  10.             }
  11.             else
  12.             {
  13.                 indexDialog = _index;
  14.             }
  15.  
  16.             DokumentDialog dialog( model, indexDialog, this );
  17.             if( dialog.exec() == QDialog::Accepted )
  18.             {
  19.                 if( dialog.submit() )// call mapper->submit();
  20.                 {
  21.                     if( model->database().commit() )
  22.                     {
  23.                         qDebug() << BOOST_CURRENT_FUNCTION;
  24.                         qDebug() << "\ttransaction successful commit";
  25.                     }
  26.                     else
  27.                     {
  28.                         qDebug() << BOOST_CURRENT_FUNCTION;
  29.                         qDebug() << "\ttransaction faild to commit";
  30.                         qDebug() << "\t" << model->lastError().text();
  31.  
  32.                         model->database().rollback();
  33.                         model->revert();
  34.                     }              
  35.                 }
  36.                 else
  37.                 {
  38.                     qDebug() << BOOST_CURRENT_FUNCTION;
  39.                     qDebug() << "\tsubmit from mapper error";
  40.                     qDebug() << "\t" << model->lastError().text();
  41.  
  42.                     model->database().rollback();
  43.                     model->revert();
  44.                 }
  45.             }
  46.             else
  47.             {
  48.                 model->database().rollback();
  49.                 model->revert();
  50.             }
  51.         }
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First insert/update is ok. Second and next returns me an error:
Qt Code: 
  1. submit from mapper error
  2.       "invalid transaction handle (expecting explicit transaction start) Unable to execute query"
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When I comment transaction, commit, rollback then insert and updates works ok, but I want to solve transaction issue.