Accessing MS SQL UID-fields with Qt

When working with a database that relies heavily on uniqueidentifiers, I experienced problems with handling those fields with Qt's built-in SQL-classes.
First, I connect to the database via the QODBC-driver. Then I fetch the results of table 'a' and tried to fetch the corresponding results in table 'b', which are referenced by foreign keys. Here's a code-snippet:

QSqlQuery a(db);
a.exec("select id from a");
a.next();
QSqlQuery b(db);
b.prepare("select id from b where b.id_a=:id");
while(a.isValid())
{
   b.bindValue(":id",a.value(0));
   b.exec();
   // ERROR: Operand type clash: image is incompatible with uniqueidentifier
   a.next();
}

So Qt converts the binary data it received from the uniqueidentifier to a binary blob of type image, it seems.
There's a simple way to convert the GUID that is stored in a.value(0) to a formatted UID-string, which in turn can be used to bind the value of the second query.

   QString uuidToString(const QVariant &v)
   {
      // get pointer to raw data
      QByteArray arr(v.toByteArray());
      std::string result(arr.constData(),arr.size());
      assert(result.size() == 16);
      const char *ptr = result.data();
      // extract the GUID-parts from the data
      uint   data1 = *reinterpret_cast<const uint*>(ptr);
      ushort data2 = *reinterpret_cast<const ushort*>(ptr+=sizeof(uint));
      ushort data3 = *reinterpret_cast<const ushort*>(ptr+=sizeof(ushort));
      uchar  data4[8] =
      {
         *reinterpret_cast<const uchar*>(ptr+=sizeof(ushort)),
         *reinterpret_cast<const uchar*>(++ptr),
         *reinterpret_cast<const uchar*>(++ptr),
         *reinterpret_cast<const uchar*>(++ptr),
         *reinterpret_cast<const uchar*>(++ptr),
         *reinterpret_cast<const uchar*>(++ptr),
         *reinterpret_cast<const uchar*>(++ptr),
         *reinterpret_cast<const uchar*>(++ptr)
      };
      // create a uuid from the extracted parts 
      QUuid uuid(
         data1,
         data2,
         data3,
         data4[0],
         data4[1],
         data4[2],
         data4[3],
         data4[4],
         data4[5],
         data4[6],
         data4[7]);
      // finally return the uuid as a QString
      return uuid.toString();
   }

Using this function, you can easily bind the values to the second query:

b.bindValue(uuidToString(a.value(0)));

Edit:Starting from Qt 4.4.0 (I used the latest snapshot) QVariant supports GUIDs and hence this function fails AND is unneccessary.