In every generated migration files by South manage.py schemamigration, there usually will be a subclass of the south.v2.SchemaMigration. The class would contain forwards() and backwards() methods and also models attribute. How does South use the models attribute?
This is called ORM freezing.
We also use a human-readable format that’s easy to change; since South relies on the frozen models not only for reacreating the ORM but also for detecting changes, it’s really useful to be able to edit them now and again (and also serves as a valuable debugging tool if you attach failing migrations to a ticket).
http://south.aeracode.org/docs/ormfreezing.html
Related
I want to know if any one could give a complete list of things which need to be done when we want to remove a model from Django. I know that a similar question was asked. But it seems to be several years ago, when people were still using South to deal with Database. So I expect an answer for the recent version of Django.
I conclude what I know as follows:
Delete the codes for model from model.py
Make sure that no other file imports this model or uses them
(admin.py, views.py, etc)
Run makemigrations and migrate commands
Since Django doesn't clean database for you, you delete the table of
this model manually from you database
Also note that there is a table called ContentTypes, which keeps
records about the info our your every model. So you need to delete
the record for this model manually (But I don't know how to do it
exactly. Would any one give some explanation?)
These are all the things I know. Is there anything wrong? And did I forget anything? Maybe I'm over-cautious, but I'd like to keep the database clean.
Thanks a lot!
In Django 1.7, this is actually much simpler than you think. Let's say you have an app, books, with two models: Book and BookReview. You want to remove the Book model.
Remove all references to the Book model in your code. For example, remove the ForeignKey('books.Book') field on the BookReview model. There is no need to make a separate migration for this change.
Remove the code for the Book model from books/models.py. Now, create a migration (manage.py makemigrations). If you look at the migration that is generated, it should include a migrations.DeleteModel operation.
Run the auto-generated migration (manage.py migrate), and you should be asked about the relevant ContentType objects that are no longer needed:
Running migrations:
Applying books.0002_auto_20150314_0604... OK
The following content types are stale and need to be deleted:
books | book
Any objects related to these content types by a foreign key will also
be deleted. Are you sure you want to delete these content types?
You probably do want to delete the content types. If you don't want to be asked for input, you can use manage.py migrate --noinput.
The DeleteModel operation in this migration will drop the books_book table in your database, so you don't have to worry about manually cleaning up at all.
I have defined some models within an application, call it "blog".
djangoproject
/blog
models.py
I now want to change the models location, for example put them here:
djangoproject
/blog
xxx
/all_models
models.py
From the code point of view, this is pretty trivial, but the I guess there will be problems on the database since all the tables Django and South created are now called blog_posts blog_comments, Django relies on this naming convention and I don't want to lose the data already present in the database.
How to do this?
The easiest thing is not to bother changing the tables at all, but the code. Inside the Meta class of each of your models, put the declaration db_table = "blog_tablename", and Django will find them without problems.
You can solve this in two ways.
The first and easier one is to provide a db_table in Meta class of each of your models. The other is create a migration to apply the change.
As far as I know south doesn't support table rename, so you should do it as a three way migration:
Move de models, and create migration (now you have both tables old and new)
Create a data migration and iterate over the former table, copying objects to later
Remove the former model, and create a migration for it.
You can read a little bit more about the second way in south docs
I'm using South for migrations. I can't seem to work out if South can also be used to amend the models themselves. Upon returning to a previous model state, am I to manually alter the code?
Thanks
South does not modify your models.py. It only alters the database. It is generally used in conjunction with version control software (such as git) that would allow you to revert your models.py to match the south migration.
am I to manually alter the code?
Yes. South altering models code seems for me not like a good idea. Only you know what where exact code changes because of your migration.
And for me, I performed backwards migration couple times and in most cases didn't wanted the model to change to previous state.
I am writing an app in Django which has a few sitewide fixed objects I need to instantiate which, for one reason or another, can't be fixtures.
For example, some (e.g. Permissions) I can't hardcode a PK for because I can't be sure that they'll be loaded in a certain order (and if I use pk=null then I get an IntegrityError if they exist already.)
Others (e.g. Sites) depend on values in settings.py. Edit: These need to run each time the project is deployed, otherwise I could use South's data migrations as per super9's suggestion below.
Since these are Django models, they're not directly related to any of the apps in my project. It would make the most sense to load them in settings.py but that results in a circular import. It works in urls.py but putting the loading code there seems hackish and out-of-place.
I looked into hooking a receiver into post_syncdb as follows:
#receiver(post_syncdb)
def create_groups_and_permissions(sender, **kwargs):
print "Creating groups and permissions"
u_ct = ContentType.objects.get(model="user")
Group.objects.get_or_create(name='Group 1')
Permission.objects.get_or_create(name="Perm 1", codename="perm_1", content_type=u_ct)
However, since I'm using South, per the documentation it only sends post_syncdb when the table is first created. I could call syncdb manually after each time I migrate but would prefer not to.
I am nearly resolved to put them in urls.py or the most closely related app's models.py but thought I'd check here to see if there's an accepted way of loading fixed objects which can't be loaded as fixtures.
Have you checked out data migrations in south yet? http://south.aeracode.org/docs/tutorial/part3.html
Sounds like it might be what you need.
How does Django handle changes to my Model? Or, what help does it offer me to do this?
I am thinking of a situation where I have already have published data to my DB which I don't want to lose, but I need to make changes to my data model - for example, adding extra fields to a particular class, changing the types of fields, etc. My understanding is that syncdb won't ever alter tables that already exist in the DB.
For example, let's say I have the following model:
class Person(models.Model):
name = models.CharField(max_length=200)
phone_number=models.CharField(max_length=200)
hair_colour=CharField(max_length=50)
Things I might want to do to Person off the top of my head:
I wish to add an 'age' field.
I realise I want to use IntegerField instead of CharField for phone_number (whether this is a good idea or not, is out of scope...) - assuming it's possible.
I realise that I no longer wish to define hair_colour 'inline' within Person, because several people share the same hair colour - I wish instead to change this to be a foreign key to some other model.
Whilst I can imagine some of these are tough/impossible for the framework to 'guess' exactly what needs to be done to my data if all I do is update the models.py, I can imagine that there might still be some tooling to help enable it - does it exist?
In particular I imagine there must be some good patterns for option 1.
I'm very new to Django and have no experience with any other ORM-type stuff, which I think this is - I've always been a bit suspicious of ORMs, mainly for the reasons above :)
Django itself will not attempt to modify an already created database table. What you are trying to do is typically called "Migration" and there are a couple of different Database Migration tools available for Django.
South
Schema Migrations
Data Migrations
Backwards Migrations
Nash Vegas
Schema Migrations
Data Migrations
Django Evolution
Schema Migrations
Data Migrations (Unknown)
Backwards Migrations (Unknown)
Of the three South is probably the most widely used but they each have different ways of dealing with migrations. You can see more details on the comparison on Django Packages.
Much of what you're asking about can be done with the django project South. You add it as an INSTALLED_APP. Create a baseline, then as your model changes it creates SQL statements to convert your tables and the rows with-in the tables to the new model format.