Returning a QSqlQuery result from a QThread - c++

Question

I am accessing a MySQL 5.6 database using Qt 5.3.1 SQL module. Currently I try to move some of that code from the main thread to a custom thread to allow the GUI thread to stay responsive during DB updates.
I understood that everything (including establishing of the connection) must be moved to the custom thread. I am using queued signals and slots to achieve this and it works properly.
However there is one thing I am not sure about: How can I return query results back to main thread? Of course I will use a signal for that. But what kind of object should I return in that signal?
Should I return the QSqlQuery? I suppose this will be dangerous since QSqlQuery is attached to the connection/database in some way.
Should I return a list of QSqlRecord objects taken from the query using record()? Unfortunately the documentation does not say a word if this is safe.
What is the right container/way to safely return the results?

Answer 1

If, for example, the database contained personal details, you could create a separate class, derived from QObject: -
class Person : public QObject
{
Q_OBJECT
public:
Person();
private:
QString m_firstName;
QString m_surname;
QString m_address
QDateTime m_dateOfBirth;
};
Then, having registered its metadata for using with signals and slots, retrieve the database record, populate the Person object and send it with signals and slots. The classes you create can then represent the tables in the database.
However, a much simpler method would be to use a QMap and emit a signal with that instead: -
QMap personMap;
personMap["name"] = sqlRecord.value().toString("name");
personMap["surname"] = sqlRecord.value().toString("surname");
personMap["address"] = sqlRecord.value().toString("address");
...etc
It may be a good idea to emit a function that takes a token and the map, where the token denotes the type of information that the map contains:-
emit RetrievedData("Person", personMap);
I would avoid sending the SqlRecord or anything to do with the underlying method of storing the data. It's always good to use loosely coupled classes. This way, you could decide to replace the database storage with another mechanism, without having to refactor all the other code.
----------- In response to comments ------------
Populate a map with the sql record. For simplicity, we assume all returned items are strings.
If record items are numbers, simply convert to string before storing in the map.
QMap PopulateMap(SQLRecord& sqlRecord)
{
QMap map;
for(int i=0; i<sqlRecord.count(); ++i)
{
map[sqlRecord.fieldName(i)] = sqlRecord.value(i).toString();
}
return map;
}