Regression: many-to-many fields are not being correctly persisted to db from the admin

[Adrian Capitanu]

Prerequisites:

• database: postgres16
• settings.py file that points to it
• have new connection pooling configured
• have a model that has a m2m field
• register that model to admin
• create a couple of instances to use in the m2m relationship

Steps to repro:

1. via admin, on a model with an m2m field, create a new model instance with no m2m relationships and save
2. edit the same instance, adding extra m2m relationships and save
3. re-open the same instance and observe m2m field

Repro rate:

• inconsistent
• maybe related to connection pooling?

Expected result:
The m2m field should have the options selected at (2) if the save was successful.

Current result:
The m2m field does not have some or all the options selected at (2), despite the save being successful.

Affected Versions: 5.1b1

Working Versions: 5.0.7

[Tim Graham]

Does this look like a server-side (wrong data in database) or a client-side issue (options not populating correctly)? Could you try to ​bisect the regression?

[Sarah Boyce]

Hi Adrian, I appreciate you have said this is inconsistent, I am struggling to replicate.
I have tested using a Postgres connection pool ("pool": True) using 5.1b1 using a model with a m2m field registered in the admin.
I have tried to see if I can find issues client side (interacting with the plus widget, Firefox/Chrome/Edge, mobile view) but no joy.

Looking into Tims suggestions would really help.

Also you can share extra info such as

• how have you setup the connection pool
• the admin and model (in case it's a different widget for example)
• you said downgrading to 5.0.7 works (I assume this has no connection pool), does using 5.1b1 without a connection pool have this behaviour?