Trying to replace a char field with a forgein key feild

[matts...@overstock.com]

So when we started the database, it was good enough to have a char field with choices. But as time progressed I need to convert that field to a foreign key field. The follow I'm trying to take is such:

1. rename <fieldname> to tmp_<fieldname>
2. create a new field with <fieldname>
3. have a RunPython fill the data for <fieldname>
4. delete field tmp_<fieldname>

I can get steps 1 through 3 to work with out issue. But once I remove the old field from the code, the migration breaks because the field no longer exists (which makes sense). I'm curious if there is a way to work around this, because my googling is coming up empty.

[Codewise IO]

You are going to want to create an empty migrations file and edit it to make your data transformations. See this article.

https://simpleisbetterthancomplex.com/tutorial/2017/09/26/how-to-create-django-data-migrations.html

[Matthew Stroud]

Thanks for the reply.

This is a perfect coverage of steps 1 through 3 in what I'm doing, but it will break when I do step 4. I basically need to extend the model temporarily so the code doesn't break (I just thought of this so I'll try to implement it).

Thanks,
Matthew Stroud

On 8/28/18, 5:35 AM, "django...@googlegroups.com on behalf of Codewise IO" <django...@googlegroups.com on behalf of codew...@gmail.com> wrote:

You are going to want to create an empty migrations file and edit it to make your data transformations. See this article.

https://simpleisbetterthancomplex.com/tutorial/2017/09/26/how-to-create-django-data-migrations.html

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[Michal Petrucha]

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On Mon, Aug 27, 2018 at 09:37:03AM -0700, mattstroud via Django users wrote:
> So when we started the database, it was good enough to have a char field
> with choices. But as time progressed I need to convert that field to a
> foreign key field. The follow I'm trying to take is such:
>
>

> 1. rename <fieldname> to tmp_<fieldname>
> 2. create a new field with <fieldname>
> 3. have a RunPython fill the data for <fieldname>
> 4. delete field tmp_<fieldname>

>
> I can get steps 1 through 3 to work with out issue. But once I remove the
> old field from the code, the migration breaks because the field no longer
> exists (which makes sense). I'm curious if there is a way to work around
> this, because my googling is coming up empty.

I'm not following – why would the migration break? What part of it
would break, and how? None of what you wrote suggests that this should
in any way fail, as long as operations 1, 2, and 4 are performed with
AlterField, AddField, and RemoveField, and you use model classes
passed into RunPython through the ``apps`` argument.

Could you perhaps provide some more details about the failure that
you're experiencing?

Michal
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[Matthew Stroud]

Thanks for the reply.

Sadly in the process of me messing around with, I somehow fixed it. I'm not sure how, but when I get a chance I'll look through my changes and see what did it.

In the meantime, I'm going to claim user error until I can figure it out.

Thanks,
Matthew Stroud

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