Re: Migrate a many-to-many field to an explicit intermediate ("through") model?

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Hi Gergely,

On 07/23/2015 02:27 PM, Gergely Polonkai wrote:
> Yes, you are right, my attempt is not the solution to your problem; it
> seems that this m2m field really cannot be modified like this. With some
> slight modifications, though, it may be.
>
> 1) create the through table
> 2) migrate data with RunPython — if you want to be portable, stay with
> RunPython instead of RunSQL

Yes, this is true. Don't use RunSQL if you want your code to be portable
between database backends. (For most of my non-reusable-apps code, I
take great advantage of Postgres-specific features and don't care at all
about cross-database portability.)

> 3) delete the ManyToManyField
> 4) recreate the ManyToManyField with the through option
>
> All this can (and I think, should) go into the same migration file.

Hmm. I'm almost sure I've had problems in the past with trying to do
schema alterations and data migration in the same migration file (thus
same transaction), but with a simple test just now it seems to work OK
on Postgres. I'll have to start trying to combine migrations like this
more often and see how it goes.

> Meanwhile it might worth a question to the devs (or a more thorough
> search on the topic) to understand why you can't switch from a simple
> m2m field to one with a through option.

Just because it's a bit complex to implement, and nobody has implemented
it yet.

Carl

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