Changing PK data type with Django and South.py

# encoding: utf-8
import datetime
from south.db import db
from south.v2 import SchemaMigration
from django.db import models

class Migration(SchemaMigration):

    # This can get stupid. When running all migrations from scratch such as
    # during tests then dependant apps will need to be initialised before this
    # migration otherwise there will be a data type mismatch. But all those apps
    # will need to depend on 0001_inital from this app (or put needed by in
    # there).
    depends_on = (
        ('downloads', '0001_initial'),
    )

    def get_fks(self, table, column, db_name=None):
        if db_name is None:
            # We need the database name or things go crazy in tests or other
            # cases where the database user might have access to more than one
            # database.
            if db.backend_name == 'mysql':
                db_name = db.execute('SELECT database()')[0][0]
            elif db.backend_name == 'postgres':
                db_name = db.execute('SELECT current_database()')[0][0]

        if db.backend_name == 'mysql':
            query = """
                SELECT DISTINCT
                    table_name, column_name, constraint_name, referenced_table_name, referenced_column_name
                FROM
                    information_schema.key_column_usage
                WHERE
                    referenced_table_schema = '%s'
                    AND referenced_table_name = '%s'
                    AND referenced_column_name = '%s'
            """
        elif db.backend_name == 'postgres':
            query = """
                SELECT DISTINCT
                    kcu.table_name, kcu.column_name, kcu.constraint_name, ccu.table_name, ccu.column_name
                FROM
                    information_schema.table_constraints AS tc
                    JOIN information_schema.key_column_usage AS kcu ON tc.constraint_name = kcu.constraint_name
                    JOIN information_schema.constraint_column_usage AS ccu ON ccu.constraint_name = tc.constraint_name
                WHERE
                    tc.constraint_type = 'FOREIGN KEY'
                    AND kcu.table_catalog = '%s'
                    AND ccu.table_name = '%s'
                    AND ccu.column_name = '%s'
            """
        fks = db.execute(query % (db_name, table, column))
        child_fks = []
        for fk in fks:
            child_fks = child_fks + self.get_fks(fk[0], fk[1], db_name)
        return list(fks) + child_fks

    def get_and_drop_fks(self, table, column):
        fks = []
        if db.backend_name in ('mysql', 'postgres') and not db.dry_run:
            fks = self.get_fks(table, column)
            for fk in fks:
                if db.backend_name == 'mysql':
                    query = 'ALTER TABLE %s DROP FOREIGN KEY %s'
                elif db.backend_name == 'postgres':
                    query = 'ALTER TABLE %s DROP CONSTRAINT %s'
                db.execute(query % (fk[0], fk[2]))
        return fks

    def update_fks(self, fks, field):
        for fk in fks:
            # This loses whether or not a field allows NULL.
            db.alter_column(fk[0], fk[1], field)
        # Add FKs after all columns have been altered to prevent data type
        # conflicts.
        for fk in fks:
            db.execute(
                'ALTER TABLE %s ADD CONSTRAINT '
                '%s FOREIGN KEY (%s) '
                'REFERENCES %s(%s)'
                % (fk[0], fk[2], fk[1], fk[3], fk[4])
            )

    def forwards(self, orm):
        
        fks = self.get_and_drop_fks('shop_order', 'id')
        db.alter_column('shop_order', 'id', self.gf('django.db.models.fields.CharField')(max_length=16, primary_key=True))
        self.update_fks(fks, self.gf('django.db.models.fields.CharField')(max_length=16, primary_key=True))

        fks = self.get_and_drop_fks('shop_credit_note', 'id')
        db.alter_column('shop_credit_note', 'id', self.gf('django.db.models.fields.CharField')(max_length=16, primary_key=True))
        self.update_fks(fks, self.gf('django.db.models.fields.CharField')(max_length=16, primary_key=True))



    def backwards(self, orm):
        
        fks = self.get_and_drop_fks('shop_order', 'id')
        db.alter_column('shop_order', 'id', self.gf('django.db.models.fields.AutoField')(primary_key=True))
        self.update_fks(fks, self.gf('django.db.models.fields.AutoField')(primary_key=True))

        fks = self.get_and_drop_fks('shop_credit_note', 'id')
        db.alter_column('shop_credit_note', 'id', self.gf('django.db.models.fields.AutoField')(primary_key=True))
        self.update_fks(fks, self.gf('django.db.models.fields.AutoField')(primary_key=True))