[Interest] Qt Sql BLOB data converting

Scott Bloom scott at towel42.com
Mon Nov 28 20:11:49 CET 2022

@Christian Ehrlicher you beat me to the punch.

If you dump out the value (not the size) you will see the QVariant is a of type QString not QByteArray

        const auto value = query.value(0);
        qDebug() << "value = " << value;
        auto tmp = QVariant( value.toByteArray() );
        qDebug() << "tmp = " << tmp;

value =  QVariant(QString, "v10I-F½L\u009C{░?x\u009B)δ\u001EG#?????fóu??H")
tmp =  QVariant(QByteArray, "v10I-F\xC2\xABL\xC2\x9C{\xC2\xB0\xEF\xBF\xBDx\xC2\x9B)\xC3\xAB\x1EG#\xDE\xA6\xEF\xBF\xBD\xEF\xBF\xBD\xEF\xBF\xBD\xEF\xBF\xBD""f\xC2\xA2u\xEF\xBF\xBD\xEF\xBF\xBD\xC4\xA6")

This is the root cause of the problem.

Ive hit this before, the query.value function should return a QByteArray type of QVariant, not a QString type when the column type is of type blob.

    auto record = db.record( "test" );
    qDebug() << "record =" << record;

returns the correct record type

record = QSqlRecord(1)
 0: QSqlField("value", QByteArray, tableName: "test", required: yes, generated: yes, autoValue: false, readOnly: false) ""

Whats interesting, is in my qsql_sqlite.cpp (the driver for the sql database, 

sqlite3_column_decltype16 returns "BLOB" but sqlite3_column_type returns SQLITE_TEXT

in the init columns function, to determine which Qt type to use, it first gets the string name of the column, if empty then use  sqlite3_column_type otherwise it uses qGetColumnType

So the Record is correct, but the return type is of QString not QByteArray


-----Original Message-----
From: Interest <interest-bounces at qt-project.org> On Behalf Of Christian Ehrlicher
Sent: Monday, November 28, 2022 10:41 AM
To: Yauheni Pervenenka <yauheni.pervenenka at viber.com>; interest at qt-project.org
Subject: Re: [Interest] Qt Sql BLOB data converting

Am 28.11.2022 um 19:16 schrieb Yauheni Pervenenka via Interest:
> Thiago, hello, thanks for your reply, sorry have no instructions for 
> create and insert, but have data base with one value which can't be 
> parsed correct(db is cut to one value for provide example)
> https://bugreports.qt.io/browse/QTBUG-108992
Your database looks strange to me - even though your column type according to the schema is BLOB:

sqlite> .schema
         "value" BLOB NOT NULL

the return value of sqlite3_column_type() is SQLITE_TEXT (3).

Therefore the return value is treated as string and the data is screwed up. If I change the return value to SQLITE_BLOB (4) in the debugger, all is correct:

qDebug() << value.toByteArray().toHex();




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