postgresql - Django ORM — Significant database alterations / data migrations -


i have django powered website , ec2 postgresql server. website has growing community , tons of user submitted content. during development models , views have gotten extremely messy tack on new features. start fresh, rewrite whole program , split of models , views modular apps.

just example, if wanted migrate from:

content.models.py  class content(models.model):     user = models.foreignkey(user)     post = models.charfield(max_length=500)     photo = models.imagefield(upload_to='images/%y/%m/%d')

to:

content.models.py  class content(models.model):     user = models.foreignkey(user)     post = models.charfield(max_length=500) 
photo.models.py      photo = models.imagefield(upload_to='images/%y/%m/%d')     content = models.foreignkey(content.models.content)

what best way go without losing data?

this case solved 3 south migrations:

  1. [schema migration] create photo table
  2. [data migration] each content record create photo record photo , content fields set
  3. [schema migration] delete field photo content table

update: step 2

python manage.py datamigration <app_name> copy_photos_to_separate_model

a new file created in <app_name>/migrations/####_copy_photos…

edit file. edit forward , backward methods. first 1 called when migrating forward, other while migrating backwards.

the first 1 creates separate photos out of consolidated model. other have pick 1 of possibly many photos @ time , squeeze content model.

the special orm object represents db's state @ time of migration (despite how models.py looks in time – when deplying production different how looked when migration run on test/develop environment).

def forward(self, orm):     content = orm['<app_name>.content']     photo = orm['<app_name>.photo']     content in content.objects.all():          photo.object.get_or_create(content=content,                                      defaults={'photo': content.photo})

depending on how big table can try optimize number of queries.

it blow out if there multiple photo records per single content in db, should not case.


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