Convert QSqlQueryModel Data into QVector(s) - c++

Question

If we get data from an SQL database using the following QT functionality
QSqlQueryModel *model = new QSqlQueryModel;
model->setQuery("SELECT name, salary FROM employee");
it states in the accompanying material we can access individual elements using either of the following commands
int salary = model.record(4).value("salary").toInt();
or
int salary = model.data(model.index(4, 2)).toInt();
and that we can insert this data into a QTableView using
QTableView *view = new QTableView;
view->setModel(model);
And that this data can also be However, I can see anywhere how to extract a whole column of data or range of columns / rows (or as in the terminology of QSqlQueryModel index & roles).
Ultimately what I would like to do is efficiently extract parts of the data into either QVectors e.g column 3 of the data. or possibly into a matrix library such as armadillo or Eigen using some sort of getDataFunc(QVector rows, QVector columns).
The data I am working is can be huge, so efficiently is important and thus using a naive set of for loops seems a very bad idea.
Any help much appreciated.

Answer 1

I would suggest to have a look on QAbstractProxyModel. Especially considering that you've got a huge datasets, so copying them in between different places doesn't look good to me.
So in the code language I think I would create something like:
class RowsInColumnProxyModel: public QAbstractProxyModel {
....
void setTargetColumn( int column );
void setRowsRange(int minRow, int maxRow);
int rowCount(..);
int columnCount(..);
... whatever else on your taste from QAbstractItemModel ...
virtual QModelIndex mapFromSource(const QModelIndex & sourceIndex) const;
virtual QModelIndex mapToSource(const QModelIndex & proxyIndex) const;
}
you might want to have a look in the Qt documentation, since there are plenty examples available. Idea is that inside mapFrom and mapTo functions you provide a logic which maps
index in your column into index in full dataset, as well as proper values for rows() and columns().. rest is done automatically for you and as soon as ProxyModel is just a model you can use it in any UI controls or any other way you like. I use proxy models a lot and it's very convenient thing once you get used to it.
I will try to make some example, hope it will make things clear for you...
A model in Qt (whatever type it has), is a set of data. Depending on type it could be either a table with rows and columns (obviously model with one column is a list), or something more advanced like tree where you could have more complex hierarchical structures, but let's leave it for now.
So, QSqlQueryModel is a flat table with rows and columns, where rows - is a number of records in result set and columns are number of columns you fetched with your query..
Let's say you've got something like
Col1 Col2 Col3
1 2 a
4 3 v
5 f f
from math prospective point of view you can call it a matrix.
QModelIndex - is a class which represents a "position" in model. To make things simple you can think of it as something representing pair { row, column }
so you can fetch data from model using method
QVariant data( QModelIndex, DisplayRole)
this method will return you a content of a cell located at position described by QModelIndex. To convert physical row and column you should use method
QModelIndex index(int row, int column, QModelIndex & parent = QModelIndex());
you can ignore last parameter for now (it's only used on tree models), so to get any cell from the flat table model (which QSqlRecordModel is) you can use code like:
QVariant value = model.data( model.index( row, column) );
int intValue = value.toInt();
.. or in case you expect a string
QString strValue = value.toString()
.. etc
so, getting close to subject... you task is to extract a vector, so lets say a part of any matrix column...so you define a class like:
class Vector: public QAbstractProxyModel {
protected:
// make them =0 in constructor
int m_column;
int m_minRow;
int m_maxRow;
...
void setTargetColumn( int column ) {
// emitting signals important to let rest of the world now about changes
emit beginResetModel();
m_column = column;
emit endResetModel();
}
void setRowsRange( int minRow, int maxRow ) {
// emitting signals important to let rest of the world now about changes
emit beginResetModel();
m_minRow = minRow;
m_maxRow = maxRow;
emit endResetModel();
}
virtual int columnCount(const QModelIndex & parent = QModelIndex()) {
return 1; // since it's vector
}
virtual int rowCount(const QModelIndex & parent = QModelIndex()) {
return m_maxRow - m_minRow;
}
.. now most exciting part..
virtual QModelIndex mapFromSource(const QModelIndex & sourceIndex) const {
int sourceRow = sourceIndex.row();
int sourceColumn = sourceIndex.column();
int targetColumn = 0; // only one in vector
int targetRow = sourceRow - m_minRow;
return modelIndex( targetRow, targetColumn );
}
virtual QModelIndex mapToSource(const QModelIndex & proxyIndex) const {
// same as above but a bit shorter
return index( proxyIndex.row()+m_minRow, m_column );
}
}
ok, we've got a class let's use it
QSqlRecordModel * sourceModel = ... (so, it's something)
Vector * myVector = new Vector();
myVector->setSourceModel( sourceModel );
myVector->setTargetColumn(3);
myVector->setRowsRange(5, 10);
...
for (int i=0;i<myVector->rowCount();i++) {
int vectorItem = myVector->data( myVector->index(i,0) ).toInt();
}
hope it explains a bit more, what I meant with my answer