Opened 9 years ago

Closed 9 years ago

#26304 closed Bug (fixed)

Unable to use unmanaged ManyToMany through models when backed by a View.

Reported by: Matthew Schinckel Owned by: nobody
Component: Database layer (models, ORM) Version: dev
Severity: Normal Keywords:
Cc: Triage Stage: Accepted
Has patch: yes Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description

Given an unmanaged ManyToMany through model, (i.e., where Meta.managed is explicitly marked as False), running tests attempts to call "sqlflush", and truncate this table.

However, I have a situation where this model is actually being backed by a (Postgres) View, which cannot be truncated.

It seems that https://github.com/django/django/blob/53ccffdb8c8e47a4d4304df453d8c79a9be295ab/django/db/backends/base/introspection.py#L83 shows that all many-to-many fields are added for flushing, ignoring managed status.

I believe that this can be resolved by only adding those many-to-many fields that do not have a through model marked as unmanaged.

Change History (7)

comment:1 by Matthew Schinckel, 9 years ago

Oh, in case it's not clear, this is an unmanaged through model between two managed models.

Otherwise https://github.com/django/django/blob/53ccffdb8c8e47a4d4304df453d8c79a9be295ab/tests/unmanaged_models/models.py#L99-L105 would show a failure...

comment:2 by Simon Charette, 9 years ago

Has patch: set
Patch needs improvement: set
Triage Stage: UnreviewedAccepted
Version: 1.9master

in reply to:  description ; comment:3 by Markus Holtermann, 9 years ago

I believe that this can be resolved by only adding those many-to-many fields that do not have a through model marked as unmanaged.

Don't you run into an issue if the rows on either end of the M2M field are truncated but the rows in the through table still exist pointing to nowhere, which effectively would blow up with broken foreignkey constraints?

in reply to:  3 comment:4 by Matthew Schinckel, 9 years ago

Replying to MarkusH:

I believe that this can be resolved by only adding those many-to-many fields that do not have a through model marked as unmanaged.

Don't you run into an issue if the rows on either end of the M2M field are truncated but the rows in the through table still exist pointing to nowhere, which effectively would blow up with broken foreignkey constraints?

In my case, it is a View that is acting as the model source: which cannot _ever_ be truncated. In this particular instance, it's something very much like http://schinckel.net/2016/01/27/django-trees-via-closure-view/

That's not to say you are incorrect, I think there may still be situations such as you describe: I guess there could be cause to use an unmanaged through model with managed models at either end, that was actually a table.

Last edited 9 years ago by Matthew Schinckel (previous) (diff)

comment:5 by Simon Charette, 9 years ago

Patch needs improvement: unset

comment:6 by Markus Holtermann, 9 years ago

Yes, in your case. I'm thinking more about the general case if we "just don't add the m2m fields". I think we should check if the through field "is a table" / "is not a view".

comment:7 by Tim Graham <timograham@…>, 9 years ago

Resolution: fixed
Status: newclosed

In 60633ef:

Fixed #26304 -- Ignored unmanaged through model in table introspection.

Note: See TracTickets for help on using tickets.
Back to Top