This is more for me than the end user, because if there is an error with
a query, there is—almost aways—nothing a user can do, since it is
probably an error in the static SQL string or the database. However, it
is better to show an error, than to do nothing at all when there is a
failure.
According to Qt’s documentation[0], QFuture relies on exceptions for the
error handling. At first i assumed that i had to attach an onFailed
handler to QFuture in order to receive that exception and skip the then
handler.
However, i can not attach the onFailure inside Database::query, before
returning the QFuture, because the subsequent then is only executed when
there _is_ an error, never in the normal, non-exceptional, case. I
would have to add the onFailure after then.
Nonetheless, i found out that there is no need for onFailure: since i do
not call result(), i do not get the exception, and the actual work is
performed in the then handler when no exception is raised.
[0]: https://doc.qt.io/qt-6/qfuture.html
To query the database, i have to run the query inside the same thread
where the database was created, which means that Database should be a
singleton not only within QML, but also in C++, and has to be the _same_
singleton in both worlds.
Although i expose an object that i have created, i followed the same
section titled “Exposing an existing object as a singleton” from Qt’s
documentation[0]. The only difference is that i do not have to declare
the element as a foreign type, because it is a bona fide QObject.
[0]: https://doc.qt.io/qt-6/qml-singleton.html#exposing-an-existing-
object-as-a-singleton
I only want to store these options if the connection to the database is
established, to avoid saving incorrect values by error. I, then, did not
use property alias, as the documentation reccomends, and instead have a
function to save the parameters when the application sees the database
open.
I have to use the function because a Connections inside LoginPage gets
called _after_ the Connections in Main, meaning that i would get deleted
before it has the chance to save the settings.
Now i can’t assume connect options will include search_path, as it is
unreasonable to request people to know the internal schemas of the
application.
Since i only plan to use a single connection, i can use QtSql’s default
connection name.
According to the documentation, i have to make sure that no query or
database object is open when i remove the database. I now use C++
scopes for that, but i need to have a QString declared outside of it to
get the default connection’s name.
I want to perform all SQL queries in a thread, to avoid freezing the UI,
that sometimes might happen when there is a lot of data to fetch; should
not happen very often, though.
Neither libpq nor Qt SQL allow queries on the same connection from
differents threads, and, in Qt SQL, all queries must be performed from
the same thread where the connection was established. In Qt5 i had to
either create a connection per thread, or use a QThread-derived object
to hold the connection and use signals and slots to pass query and
response data between the UI and database threads; it was usable but not
pretty.
With Qt6 and Concurrent’s QThreadPool now i can use QFutures instead,
that are not as cumbersome as with Qt5, because i no longer need
QFutureWatcher. I still have the problem that all queries must be done
from within the same thread, and QThreadPool uses an arbitrary thread.
The solution is to create a “pool” with a single, non-expirable thread,
and call all Concurrent::run onto that pool.
I have to test it properly, and first need to open the database to test
whether that, at least, works. I added a simple “login page” for that,
and to make a first attempt to error messages; i use a control that is
like Kirigami’s InlineMessage for now, but i am not sure.
I also do not know how i will configure database’s connection details. I
usually make use of pg_service.conf, because then the application only
need to know its service name, but i am not sure whether other people
would find it as comfortable as i do.
jordi fita mas