QSqlTableModel

[VRonin]

• You are leaking sqlTableModel
• sqlTableModel->select(); returns a boolean you can use to check if it worked or not
• "Do you mean that I should change this on this?" No, you should remove QSqlDatabase db; from dialog.h and add auto before db = QSqlDatabase::addDatabase("QSQLITE");

[Xilit]

@VRonin
sqlTableModel->select(); returns a boolean you can use to check if it worked or not

Hmmm... It is really doesn't work. It returns false. So what can I do with is? There is nothing in docs about it. It just says:

returns true if successful; otherwise returns false.

But how can I fix it? What went wrong?

[Chrisw01]

@Xilit Take a look at lastError(); to see if there is any errors available.

From the Docs:

[virtual]void QSqlTableModel::setTable(const QString &tableName)
Sets the database table on which the model operates to tableName. Does not select data from the table, but fetches its field information.

To populate the model with the table's data, call select().

Error information can be retrieved with lastError().

Thanks..

[Xilit]

@Chrisw01

Thank you!

db.lastError().text() is empty string, db.isValid() returns true.

The problem is here:

if(sqlTableModel->select())
            {
                QMessageBox::information(this,"Info","Selected!");
            }else{
                QMessageBox::critical(this,"Error","Failed!");
            }

And I always get "Failed!". Why? It seems that my DB is valid, it is file, it is opened but it can't select data from database?

[SGaist]

Hi,

Check the lastError of your QSqlTableModel.

[Xilit]

@SGaist

Thank you! I've already checked that.(

Problem solved! I've got updated table with another table name. So I just update information in .cpp and all is working now! Thank you all for helping me out!

BTW, are there any possibility to display double number without exponential format in QSqlTableModel? I can use something like this QString::number(result, 'f', 2), but how can I select all date from colomn "Population"? Should I use SqlQuery / .next() instead of this

            sqlTableModel->select();
            ui->tableView->setModel(sqlTableModel);

Thank you!

[JonB]

@Xilit
Yes, use number format to select desired format!

You already have all rows read in. model->data(model->index(row, column)) gives you the column data in a row, e.g. sqlTableModel->data(sqlTableModel->index(0, 1)), sqlTableModel->data(sqlTableModel->index(1, 1)) for the two items in your picture.

[Xilit]

@JonB

Hi! Glad to see you!

I've tried this

QModelIndex index = sqlTableModel->index(0,3);
sqlTableModel->setData(index, QString::number(sqlTableModel->data(index).toDouble(), 'f', 3));
ui->tableView->setModel(sqlTableModel);

But it nothing change. It still display in exponential format.

[JonB]

@Xilit
Don't do this via setData(). You don't want to set/change any data, only the way it is displayed.

(BTW, my guess here is that if you looked at the return result of your setData() [which you ought always do] you might see yours is failing, trying to store a string in a table column known to be a number.)

Sub-class from Qt-supplied QSqlTableModel. Override the data() method, and have it return the desired conversion to formatted string in whatever columns are numeric to be displayed like that. Do so for the (default) DisplayRole case only.

That's how I believe in doing it. I think there are some who would do the conversion/display in a QStyledItemDelegate instead.

In general, you should only need to set the view's model once at the start, not each time a value changes. However if you change as above you won't be changing any values anyway.

[Xilit]

@JonB

Thank you! I'll try to do that.

I think there are some who would do the conversion/display in a QStyledItemDelegate instead.

You know, when I write previous comment I thought about using QStyledItemDelegate . If you say so maybe I realy should use it.

In general, you should only need to set the view's model once at the start, not each time a value changes. However if you change as above you won't be changing any values anyway.

So if I want to change some values I need to derived from abstractModel and override methods, right? Ok, if I derived from QAbstractTableModel and reimplement rowCount(), columnCount(), data(), setData() and flags() in my countriestablemodel.cpp and pass this model to mainwindow.cpp to show. And to mainwindow.cpp I also give QStyledItemDelegate from countriesstyleditemdelegate.cpp.

Something like this:

countriestablemodel.cpp -> mainwindow.cpp (provide model with data from DB)
countriesstyleditemdelegate.cpp -> mainwindow.cpp (provide delegate to model)

So, model with data and delegate to this model must "meet" each other in mainwindow.cpp. Is this scheme correct? If you have any suggesions it will be appreciated.

Thank you!

[JonB]

@Xilit
Yes, you can do something like this.

As I said, personally I do not do this in a QStyledItemDelegate at all. I suggested you do it in data() override for Qt::DisplayRole. Personally I think this is simpler to write, but perhaps that's opinion.

Rationale: QStyledItemDelegate is only usable in a QTableView or similar. However, to me the choice to display a numeric in a particular format like you have is not restricted to/has nothing to do with whether you happen to be displaying it as an item in table. If, say, you want to display it in its own widget, or export it to a text file, you would (presumably) still want to display/output it in your chosen format. So I see it best as an attribute of the data, putting the logic in data(Qt::DisplayRole) makes that available/shareable everywhere.

[Xilit]

@JonB

Thank you! I'll try it out.)

[Xilit]

BTW is there are recommendation / tutorial how to reimplement QVariant QAbstractItemModel::data(const QModelIndex &index, int role = Qt::DisplayRole) const? To know how to reimplement method I need to know how this method works but I don't. Were I can find realization of it? There are no information about it in docs.

[JonB]

@Xilit
No mention about it in the docs?? Oh, I think you mean you have found the reference page (https://doc.qt.io/qt-5/qabstractitemmodel.html#data), but not examples? One is https://doc.qt.io/qt-5/model-view-programming.html#a-read-only-example-model, another is https://doc.qt.io/qt-5/model-view-programming.html#read-only-access. Read around those two areas. There are perhaps better links, I don't know. Understanding how data() & setData() methods work and can be overridden is important for Qt models.

[SGaist]

Did you follow the Model View Programming Guide ?
There are several examples linked with some of them implementing custom models and views.

[VRonin]

@JonB said in QSqlTableModel:

I suggested you do it in data() override for Qt::DisplayRole. Personally I think this is simpler to write, but perhaps that's opinion.

Strongly disagree. The model is data, it shouldn't care how the data is represented. Also, the model is locale unaware

Just subclass QStyledItemDelegate and override displayText:

class NoScNotationDelegate : public QStyledItemDelegate{
Q_OBJECT
Q_DISABLE_COPY(NoScNotationDelegate)
public:
using QStyledItemDelegate::QStyledItemDelegate;
QString displayText(const QVariant &value, const QLocale &locale) const override{
switch(value.type()){
case QMetaType::Double:
return locale.toString(value.toDouble(),'f');
case QMetaType::Float:
return locale.toString(value.toFloat(),'f');
default:
return QStyledItemDelegate::displayText(value,locale);
}
}
};

Then you can just use it with tableView->setItemDelegate(new NoScNotationDelegate(tableView)); (you can also use setItemDelegateForColumn to apply it ta a single column)

[JonB]

@JonB said in QSqlTableModel:
@Xilit
Prefacing my remarks to @VRonin below: Then in view of his post above, I think you should follow QStyledItemDelegate route, as he is much more of a Qt expert than I am.

@VRonin
I'll keep it brief here, otherwise perhaps we should take this issue to its own dedicated thread. I respect your expertise, but it does not chime right for me. Just a couple of reasons:

• Somewhere in the existing Qt code for data(DisplayRole) there must be the equivalent of QString::arg(char format = 'g') to give the results it does. You are saying that is OK, but changing to QString::arg(char format = 'f') is not. Makes no sense to me. If the model must not care how the data is represented, it should not have a DisplayRole.

• Your code is all in a QStyledItemDelegate for a QTableView or similar. I do not want special code for displaying a number in a view, I want it just as much for displaying it in, say, a label, or hundreds of other places. So the code has to be refactored to be available from all other places you might want your string representation of a numeric data item, and one will never be able to use plain, default data() method in practice (given something like the OP's requirement for numeric format).

[Xilit]

@JonB
@SGaist
Thank you guys for links! I'm going to dive in to deeply understand how model works.

@VRonin
Thank you that you find time to help me out with my programm and wrote example code! Written code is really very valueble for me because sometimes it's very hard to understand what senior experts talking about without code example.)

Thank you all guys that I can always count on your help!

[VRonin]

@JonB said in QSqlTableModel:

Somewhere in the existing Qt code for data(DisplayRole) there must be the equivalent of QString::arg(char format = 'g')

No, data is converted to string to represent it inside QStyledItemDelegate::displayText, nowhere else (and it uses QLocale::toString instead of the C-locale-only QString::number). The model holds the data in the native format and never cares about how it is represented to the user

Your code is all in a QStyledItemDelegate for a QTableView or similar

Yes, my code is for the model-view-delegate architecture

I do not want special code for displaying a number in a view, I want it just as much for displaying it in, say, a label, or hundreds of other places

Then it's the final user that has to take care of knowing how to convert a double to string. Exactly because of the reason you mentioned. the model is agnostic to what you use it in/for

[JonB]

@VRonin said in QSqlTableModel:

No, data is converted to string to represent it inside QStyledItemDelegate::displayText, nowhere else. The model holds the data in the native format and never cares about how it is represented to the user

If you are saying that QStyledItemDelegate::displayText does its own conversion off data(EditRole) then I don't get it, because I change data(DisplayRole) for numbers and they come out reflecting that in table views? Unless you are saying I am going mad.

If the model never cares about representation to the user, what is the purpose of DisplayRole? (Or, for that matter, FontRole etc. Indeed, why does the model's data() even offer or get called for such roles?)