[PyQt] QSqlQuery::value: not positioned on a valid record

[iMath]

After setting up a table model like this:
code is here [stackoverflow.com]

the content is displayed properly, but when I edit the data item then press enter ,this error occurred :


    QSqlQuery::value: not positioned on a valid record





when I close the application and restart it again ,I found the editing I did last time is not stored into the database ,anyone can fix my problem here .

Note:I did not use 


    model->removeColumn(0)





here.
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