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.
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 have a simple Django website (just a form, really) which asks a few questions and saves the data in a SQL database using Model.save(). Pretty simple. I want to add a model to do page counting, though -- it'll just be a single object with a field that gets incremented each time the page's view function is called.
Now, I know little to nothing about SQL. I imagine this is not terribly difficult to do, but I would like to avoid losing or breaking all my data because of a slight misunderstanding of how the database works. So how can I go about doing this? I've heard of some third-party apps that will implement such functionality, but I'd like to do it myself just for learning purposes.
I don't understand why your existing data would be affected at all. You're talking about adding a completely new table to the database, which is supported within Django by simply running manage.py syncdb. The case where that doesn't work is when you're modifying existing tables, but you're not doing that here.
I must say though that learning and using South would be of benefit in any case. It's good practice to have a tool that can maintain your model tables.
(Plus, of course, you would never lose any data, because your database is backed up, right? Right?)
Since you're adding new model, you can just run syncdb and it will create new table for your model. If you were to change existing model, then you'd need to manually update database schema using "ALTER TABLE" statements or use South instead.
Is it possible to implement 'expando' model in Django, much like Google App Engine has? I found a django app named django-expando on github but it's still in early phase.
It's possible, but it would be a kludge of epic proportions. GAE uses a different database design known as a column-based database, and the Django ORM is designed to link with relational databases. Since technically everything in GAE is stored in one really big table with no schema (that's why you don't have to syncdb for GAE applications), adding arbitrary fields is easy. With relational databases, where each table stores exactly one kind of data (generally) and has a fixed schema, arbitrary fields aren't so easy.
One possible way you could implement this is to create a new model or table for expando properties that stores a table name, object ID, and a TextField for pickled data, and then have all expando models inherit from a subclass that overrides the __setattr__ and __getattr__ methods that will automatically create a new row in this table. However, there are a few major problems with this:
First off, it's a cheap hack and is contrary to the principles of relational databases.
Second, it is not possible to query these expando fields without even more hacks, and even so it would be ludicrously slow.
My recommendation is to find a way to design your database structure so that you don't need expando models.