DATABASES = {
    'default': {},
    'auth_db': {},
}

class MultiDBRouter(object):
	def db_for_read(self, model, **hints):
		if model._meta.app_label == 'auth':
			return 'auth_db'
		return 'default'

	def db_for_write(self, model, **hints):
		if model._meta.app_label == 'auth':
			return 'auth_db'
		return 'default'

	# migrate 적용할 DB를 선택
    def allow_migrate(self, db, model):
	    if db == 'auth_db':
	        if model._meta.app_label == 'auth':
	            return True
	        else:
	            return False
	    return True

DATABASE_ROUTERS = [
    '<router_path>',
]

from django.db import migrations, models

# 현재 해당 모델에 있는 값들에 적용
# 이후에 생성되는 값들에 대한 적용은 하지 않음
# Django Python2는 migrations 파일 안에 정의한 함수들에 대한 지원이 부족
def combine_names(apps, schema_editor):
    Profile = apps.get_model('app', 'Profile')
    for profile in Profile.objects.all():
        profile.first_name = '%s %s' % (profile.first_name, profile.last_name)
        profile.save()

class Migration(migrations.Migration):
    dependencies = [
        ('app', '0001_initial'),
    ]
    operations = [
        migrations.RunPython(combine_names),
    ]

from django.db import migrations, models

class Migration(migrations.Migration):
    replaces = [('app', '0001_initial'), ('app', '0002_custom_migrations'), ('app', '0003_profile_name')]
    initial = True
    dependencies = [
    ]
    operations = [
    	# 0001_initial
        migrations.CreateModel( ... ),
        # 0002_custom_migrations
        migrations.RunPython(
            code=app.migrations.0002_custom_migrations.combine_names,
        ),
        # 0003_profile_name
        migrations.AddField(...),
    ]

from django.db import migrations, models

def combine_names(apps, schema_editor):
    pass

class Migration(migrations.Migration):
    replaces = [('app', '0001_initial'), ('app', '0002_custom_migrations'), ('app', '0003_profile_name')]
    initial = True
    dependencies = [
    ]
    operations = [
    	# 0001_initial
        migrations.CreateModel( ... ),
        # 0002_custom_migrations
        migrations.RunPython(
            code=combine_names,
        ),
        # 0003_profile_name
        migrations.AddField(...),
    ]

class ExclusiveBooleanField(models.BooleanField):
    def __init__(self, on=None, *args, **kwargs):
        if isinstance(on, string_types):
            on = (on, )
        self._on_fields = on or ()
        super(ExclusiveBooleanField, self).__init__(*args, **kwargs)

    def deconstruct(self):
        name, path, args, kwargs = super(
            ExclusiveBooleanField, self).deconstruct()
        if self._on_fields:
            kwargs['on'] = self._on_fields
        return name, path, args, kwargs

import uuid

from django.db import migrations, transaction

# Model의 값들을 Chunk로 나눠서 update
def gen_uuid(apps, schema_editor):
    MyModel = apps.get_model('myapp', 'MyModel')
    while MyModel.objects.filter(uuid__isnull=True).exists():
        # atomic을 유지
        with transaction.atomic():
            for row in MyModel.objects.filter(uuid__isnull=True)[:1000]:
                row.uuid = uuid.uuid4()
                row.save()

class Migration(migrations.Migration):
    # DDL Transaction을 지워하는 않는 DB에는 적용되지 않는다. 
    atomic = False

    operations = [
        migrations.RunPython(gen_uuid),
    ]

class Migration(migrations.Migration):
    run_before = [
        ('third_party_app', '0001_do_awesome'),
    ]

from django.apps import apps as global_apps
from django.db import migrations

def forwards(apps, schema_editor):
    try:
        OldModel = apps.get_model('old_app', 'OldModel')
    except LookupError:
        # The old app isn't installed.
        return

    NewModel = apps.get_model('new_app', 'NewModel')
    NewModel.objects.bulk_create(
        NewModel(new_attribute=old_object.old_attribute)
        for old_object in OldModel.objects.all()
    )

class Migration(migrations.Migration):
    operations = [
        migrations.RunPython(forwards, migrations.RunPython.noop),
    ]
    dependencies = [
        ('myapp', '0123_the_previous_migration'),
        ('new_app', '0001_initial'),
    ]

    if global_apps.is_installed('old_app'):
        dependencies.append(('old_app', '0001_initial'))