What is the best way to read/write objects in a file with the following conditions - sql

Question

I am willing to store objects in a database. The purpose is to be able to read / write these objects with the program. The requirements are the following:
Objects can be complex using Qt classes such as QList, QString ... or even can contain other objects that use QObjects
The database should not be readable or modified by human (no text file, and if I use sqlite database, it has to be encrypted in a way)
I should be able to remove, read an object by its name and count the number of objects in the database, without loading everything in the memory
I asked a question here, to do this with a QDataStream with a minimalist example. But it seems it is not the best way to proceed.
Would you have some suggestions regarding the solutions that exist for this purpose?
I have tried the following but it does not fulfill the requirements:
Storing text in sqlite with QtSQL: but the data is accessible by using sqlitemanager for example, can be modified or removed by humans. Moreover, I have no idea regarding the way to store QList for example or other objects that I created and contain QObject (for example, 2 QList)
Storing binary data using QDataStream: in this case, I cannot count the number of objects in my file, neither read or remove a specific object without loading the entire file in memory.
I would be grateful if you could give me some suggestions or provide example, even if the example is minimalist.

Answer 1

I finally found a solution, especially thanks to Igor Tandetnik and thanks to the topic here
I haven't quite finalized, there is a small imperfection because I have to define an object of my user class that I don't use in order to call the readFromDB function to generate my object from the db.
On the other hand, I get this error message "QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed" each time I call my database.
Anyway, it's a bit late now, and I think it might help some people so I post this minimalist imperfect code below. I'll post an update in the next few days.
Thanks again.
#include "QString"
#include "QFile"
#include "QDataStream"
#include "qdebug.h"
#include "QtSql"
#include "QSqlDatabase"
#include "qmessagebox.h"
class User
{
protected:
QString name;
QList<QString> childrens;
public:
QString getName(){ return name;}
QList<QString> getChildrens(){ return childrens;}
void setName(QString x) {name = x;}
void setChildrens(QList<QString> x) {childrens = x;}
friend QDataStream &operator<<(QDataStream &out, const User &t)
{
out << t.name << t.childrens;
return out;
}
friend QDataStream &operator>>(QDataStream &in, User &t)
{
QString inname;
QList<QString> inchildrens;
in >> inname >> inchildrens;
t.name = inname;
t.childrens = inchildrens;
return in;
}
QByteArray object2blob( const User& user )
{
QByteArray result;
QDataStream bWrite( &result, QIODevice::WriteOnly );
bWrite << user;
return result;
}
User blob2object( const QByteArray& buffer )
{
User result;
QDataStream bRead( buffer );
bRead >> result;
return result;
}
int saveToDB( const User& user )
{
QSqlDatabase myDB = QSqlDatabase::addDatabase("QSQLITE");
myDB.setDatabaseName( "file.db");
if (!myDB.open())
{
qDebug()<<"Failed to open SQL database of registered users";
}
else
{
qDebug()<<"Successfully opening SQL database of registered users";
QSqlQuery query;
query.prepare( "CREATE TABLE IF NOT EXISTS users (name TEXT, childrens BLOB)" );
if( !query.exec() )
{
qDebug() << query.lastError();
}
else
{
qDebug() << "Table created!";
query.prepare( "INSERT INTO users (name,childrens) VALUES (:name,:childrens)" );
query.bindValue(":name", name);
query.bindValue( ":childrens", object2blob(user) );
query.exec();
}
query.clear();
myDB.close();
}
QSqlDatabase::removeDatabase("UserConnection");
return 0;
}
User readFromDB( QString name )
{
User result;
QSqlDatabase myDB = QSqlDatabase::addDatabase("QSQLITE");
myDB.setDatabaseName( "file.db");
if (!myDB.open())
{
qDebug()<<"Failed to open SQL database of registered users";
}
else
{
QSqlQuery query;
query.prepare( "SELECT * FROM users WHERE name ='"+ name +"'" );
//query.bindValue( 0, name );
if ( query.exec() && query.next() ) {
result = blob2object( query.value( 1 ).toByteArray() );
}
query.clear();
myDB.close();
}
QSqlDatabase::removeDatabase("UserConnection");
qDebug()<<result.getChildrens();
return result;
}
};
////////////////////////////////////////////////////////////////
int main()
{
User u;
u.setName("Georges");
u.setChildrens(QList<QString>()<<"Jeanne"<<"Jean");
u.saveToDB(u);
User v;
v.setName("Alex");
v.setChildrens(QList<QString>()<<"Matthew");
v.saveToDB(v);
User w;
w.setName("Mario");
w.saveToDB(w);
User to_read; //to improve here
User a = to_read.readFromDB("Georges");
qDebug()<<a.getChildrens();
return 0;
}