[sqlalchemy] debugging DetachedInstanceError

[Jonathan Vanasco]

I'm trying to figure out how I got a DetatchedInstanceError


DetachedInstanceError: Parent instance  is 
not bound to a Session; lazy load operation of attribute 'domain' cannot 
proceed (Background on this error at: http://sqlalche.me/e/bhk3)


This is happening in a web application where I never call 
`dbSession.close()`. It does use zope.sqlalchemy for pyramid_tm, but 
`keep_session` is True.  I set `expire_on_commit=False` on the sessionmaker.

Any tips on the best ways to troubleshoot how/where this is getting removed 
from a session?



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Re: [sqlalchemy] SQLAlchemy Access to Native Bulk Loaders

[Benjamin Taub]

Thank you for taking the time to lay that out, Jonathan. I am not sure if
this will be a major issue for me yet or not and I would like to stay
within SQLAlchemy to the greatest extent possible. Since I'm using the
core, I think I'll try your second option. If I run into issues, I can
rethink at that time.

Thanks again!
Ben
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On Mon, Apr 20, 2020 at 12:40 PM Jonathan Vanasco 
wrote:

>
> On Sunday, April 19, 2020 at 4:44:56 PM UTC-4, Ben wrote:
>>
>>
>> These seem to be embedded in the related SQL implementations but are
>> clearly not ANSI standard. I'm not sure if that makes it disqualifying for
>> a SQLAlchemy feature request, or if anyone else could even use it, but
>> functionality like this is something that, at least for me, would make my
>> implementation more DB independent.
>>
>
> The major database servers and clients do have similar extensions to ANSI
> and the Python DBAPI here, but they're fairly different from one another in
> how they work and their input/output formats (and capabilities).  They also
> are not necessarily supported by the underlying database drivers. Pyscopg2
> has "copy_" methods on the cursor (
> https://www.psycopg.org/docs/usage.html#copy); there are several mysql
> drivers, i looked at two and neither have explicit support for this. I'm
> not sure how this would really work though - it's basically designed for
> being implemented in the C clients with local files.
>
> The "bulk loading" functionality in SqlAlchemy is largely in the realm of
> being a low-level DBAPI operations for insert statements, and SqlAlchemy's
> performance is about as close to using the driver itself - while still
> having the syntactic sugar and not needing to write to a database
> specifically.  I don't believe there is any code to invoke a database
> client to read a source file itself; I am often mistaken though as Mike has
> put a ton of work into SqlAlchemy over the years.
>
> From experience with large imports, I can tell you the following:
>
> * The best performance for large updates involves:
> 1) take the database offline to clients
> 2) drop (and copy) all the foreign key and check constraints
> 3) import your data from a file with the native client that ships with
> your database *
> 4) add the foreign key and check constraints back in
> if you feel the need to script this, you could use `subprocess` to
> invoke the database's native client on the machine
>
> * The second best performance is:
> use SqlAlchemy's bulk strategies,
> segment out the ranges of your source data file
> run multiple processes in parallel, each processing a subset of the
> source data file
>
> * The third best performance is:
>write a script that uses the SqlAlchemy ORM with the 'windowed query'
> option to iterate over a range of lines in your source file
>have the script use a 3rd party tool like Redis to mark that it has
> "checked out" sections of the source data, and has completed/not-completed
> that range
>run multiple processes in parallel, each processing a subset of the
> source data file
>
>
> In the second and third approach, you can periodically run a query or
> script to calculate the right number of processes to run.  It could be 7,
> or 20, or another number.
>
>
>
>
>
>
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>
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
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> 
> .
>

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