PyQt Digest, Vol 191, Issue 9

[Nenad Lamza]

Dennis,

I must be honest and admit I couldn't understand a word you were talking 
about.

Are you suggesting me to use psycopg2 or something else instead of 
standard PyQt QSqlDatabase, QSqlQuery, QSqlQueryModel,... And as I 
understand you suggest that I use MVC without QSqlQueryModel and 
QTableView. How, to take a few years off and make my own MVC?

Nenad


> ------------------------------
>
> Message: 2
> Date: Mon, 8 Jun 2020 09:21:15 -0500
> From: Dennis Jensen <djensen at pgcontrols.com>
> To: pyqt at riverbankcomputing.com
> Subject: Re: What is the right way to use PostgreSQL with PyQt5
> Message-ID: <02fea112-efe9-ce6f-ec39-b4988435eab7 at pgcontrols.com>
> Content-Type: text/plain; charset="utf-8"; Format="flowed"
>
> First and foremost using the PyQt database tools is completely
> unnecessary and is either at the least adding much more complexity than
> needed and at worst it is tying the front-end way to tightly to the
> back-end which is just full numerous issues.  However, on a positive
> note you can interface with PostgresSQL and any other database software
> using straight Python allowing one to build an interface to the database
> that they can connect to the GUI via a Data-Router (or Controller) thus
> in 90% or more of the cases where you are dealing with a GUI and a data
> source (be it a database or something else that supplies data) the MVC
> or (M-C/R-V) methodology is generally the best method to apply as it
> keeps things very simple and straight forward and you can later easily
> add and/or change either the front-end and/or back-end without any
> issues and these kind of changes do happen more often than one might
> suspect.  Next it also removes or at least reduces the issues you might
> be having with drivers because once you get the PostresSQL up and
> running you do not need anything more than the Python library to
> interface with it.
>
> On 6/7/2020 12:02 PM, Nenad Lamza wrote:
>> I use PostgreSQL 9.6.17 (installed on linux server on LAN) and have
>> Win10 (or Win7) clients with installed Python 3.7 or 3.8 and PyQt
>> 5.13.2, and my PyQt5 app that uses that PostgreSQL server works ok,
>> but I really don't know what I'm doing.
>>
>> I'll describe the problem. If I don't have PostgreSQL installed on Win
>> client (I don't need it because I use PostgreSQL on linux server) or
>> have installed it but not in path, my app crashes with the error:
>> "QSqlDatabase: QPSQL driver not loaded".
>>
>> So, I need some drivers. And I install PostgreSQL 9.6.x on Win client
>> and have it in path and after that everything works ok, but only if I
>> had luck and picked the right x in PostgreSQL 9.6.x version.
>>
>> The alternative is that I don't install PostgreSQL 9.6.x (why would I
>> need to install it) on client machine and just copy some dll files at
>> the app directory. If I find the right files (drivers), everything
>> works ok, but if I don't, the app crashes with the error:
>> "QSqlDatabase: QPSQL driver not loaded" or worse, recognize the
>> PostgreSQL database on server, works ok some time, but after the
>> first, second or third query it crashes.
>>
>> Some driver combinations I copy to my app directory that work with
>> PyQt 5.13.2:
>>
>> 1.
>> Timestamp                      Size             File
>> 25/02/2016  07:19         1,015,973 libiconv-2.dll
>> 07/01/2016  09:02         1,550,023 libintl-8.dll
>> 10/07/2019  20:20           145,408 libpq.dll
>>
>> 2.
>> Timestamp                      Size             File
>> 21/06/2019  14:43         1,218,048 libeay32.dll
>> 25/02/2016  07:19         1,015,973 libiconv-2.dll
>> 07/01/2016  09:02         1,550,023 libintl-8.dll
>> 10/07/2019  20:20           145,408 libpq.dll
>> 21/06/2019  14:43           276,992 ssleay32.dll
>>
>> If I install PyQt 5.14 or 5.15 (instead of PyQt 5.13.2), the app
>> crashes, and I am afraid to upgrade PyQt5.
>>
>> So, my question is:
>> How do I know which drivers I should copy to my app directory with
>> various version of PyQt5, that's the mistery to me. And also if
>> copying the drivers to my app directory is the right way at all?
>>
>> There's a fine answer here:
>>
>> https://stackoverflow.com/questions/20884010/qt5-failed-to-load-psql-driver-in-windows
>>
>> "It's quite likely to be failing because there's a dependency of
>> |qsqlpsql.dll| missing - either |libpq.dll|, or one of the libraries
>> that |libpq| build requires." ... "You should use the /same/ libpq
>> your Qt's |qsqlpgsql.dll| was compiled against if possible."
>>
>> If that's true, how should I know with what libpq library
>> qsqlpgsql.dll was compiled in. I suppose PyQt 5.13.2's qsqlpgsql.dll
>> is compiled with different version than PyQt 5.14.x's qsqlpgsql.dll or
>> even 5.15.x's qsqlpgsql.dll.
>>
>> Also, this link has some clues:
>> https://stackoverflow.com/questions/26507353/qpsql-driver-not-loaded-qt
>>
>> And generally, what's the best way to work with PostgreSQL in PyQt5.
>> It obviously doesn't work out of the box if PostgreSQL is not
>> installed on client machine, and even if the wrong version is installed.
>>
>> Nenad
>>
>