CODEWITHSUNDEEP
djangodjango-south
Ratan Kumar
Ratan Kumar

Reputation: 1650

South Data migration in django after modifing the model

I have a project with existing class model as 

class Disability(models.Model):
    child = models.ForeignKey(Child,verbose_name=_("Child"))

But with the recent architecture change i have to modify it as 

class Disability(models.Model):
    child = models.ManyToManyField(Child,verbose_name=_("Child"))

now for this new change .. ( even i have to modify the existing database for the new one ) i guess data migration is the best way to do it rather than doing it manually .

i refered this online doc

http://south.readthedocs.org/en/latest/commands.html#commands-datamigration

but it has very less about data migration . and more about schema migration .

question 1 . if i do the schema migration will this make me loose all me previous data belonging to that old model.

question 2 . Even i am tring for schema migartion it is asking this ... 

 (rats)rats@Inspiron:~/profoundis/kenyakids$ ./manage.py schemamigration web --auto 
  ? The field 'Disability.child' does not have a default specified, yet is NOT NULL.
  ? Since you are removing this field, you MUST specify a default
  ? value to use for existing rows. Would you like to:
  ?  1. Quit now, and add a default to the field in models.py
  ?  2. Specify a one-off value to use for existing columns now
  ?  3. Disable the backwards migration by raising an exception.
  ? Please select a choice: 1

Can anyone Explain the concept and difference between schema and data migration and how this can be achieved separately .

Upvotes: 2

Views: 2862

Answers (2)

Rickard Zachrisson
Rickard Zachrisson

Reputation: 985

  1. South will try to do a transaction by moving your table data to a temporary table (I could be wrong there), then restructure the table and try to add in the origin data to the new strucutre. Like this:

old_table -> clone -> tmp_table

old_table ->restructure

tmp_table.data -> table

South will look at the field types. If there is big changes it will ask what to do. For example chaning a text field to a int field would be very hard to convert :)

  1. When you remove fields you may still want to be able to convert back to an old structure, so south will need some default data to be able to create a table with the old structure.

Moving data is always an issue since you may change table structure and field type. For example how would you manually deal with data from a Char(max_length=100) to a Char(max_length=50)?

Best suggestion is to keep good backups. Also take advantage of djangos fixtures. You can save fixtures for different datastructures along with south migrations.

South will load initial_data files in the same way as syncdb, but it loads them at the end of every successful migration process

http://south.readthedocs.org/en/latest/commands.html#initial-data-and-post-syncdb

Upvotes: 1

Daniel Roseman
Daniel Roseman

Reputation: 599470

Schema and data migrations are not different options you can take to modify your table structure. They are completely different things. Of course, data migrations are fully described in the South docs.

Here a data migration will not help you, because you need to modify your schema. And the whole point of South and other migration systems is that they allow you to do that without losing data.

Upvotes: 3

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