Saturday, April 6, 2013

django-stdimage Custom Fields and South Migrations

I'm putting the finishing touches on my latest Django site (a registration site for a dog rescue charity walk) and part of the functionality is the ability for users to upload photos. Since most people will simply snap a photo on their cell phones and upload it, and since cell phones these days take some pretty large pictures, I want to resize the images to a standard size of 800x600 when they're uploaded and also create a thumbnail.

As with most things in the Python/Django world there's a library that does exactly what I need, specifically django-stdimage, which provides a custom model field to handle resizing photos and creating thumbnails.

After installing django-stdimage via pip, updating my model class to use the new form field, and then doing a South migration, I ran into this error:

 ! Cannot freeze field 'supporter.registrant.photo'
 ! (this field has class stdimage.fields.StdImageField)

 ! South cannot introspect some fields; this is probably because they are custom
 ! fields. If they worked in 0.6 or below, this is because we have removed the
 ! models parser (it often broke things).
 ! To fix this, read http://south.aeracode.org/wiki/MyFieldsDontWork


To give a little background, South used to handle automatic introspection of custom fields, but according to the explanation of why it no longer does that in newer versions of South, "... when it broke, it broke spectacularly." Fair enough! So the expectation on newer versions of South is that you'll provide the necessary information to allow South to handle the custom fields.
Giving South what it needs to do migrations involving custom fields is simple enough but since this is the first time I've had to deal with it I thought I'd help my future self and potentially others by sharing how to handle it.

There are a couple of different scenarios with South and custom fields that you can read more about in the South docs, but in the case of django-stdimage you simply have to add the location of the custom field to South's introspection rules.

In the models.py that contains the model in which you're using django-stdimage, add the following somewhere at the top of the file:

from south.modelsinspector import add_introspection_rules

add_introspection_rules([], ['^stdimage\.fields\.StdImageField'])


Now when you run the South schema migration South will know where to find StdImageField and will be able to handle the migration.

If you have more complex custom fields be sure and read the South docs since if your custom field adds new attributes or doesn't extend another class for which there are already introspection rules, you'll have to provide the introspection rules yourself as opposed to simply providing the location of the custom field class.

I'll share the link to the app in which I'm using django-stdimage and put the code for the entire app up on GitHub soon.

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