Accessing Foreign Key Values

When you access a field that’s a ForeignKey, you’ll get the related model object. For example:

>>> b = Book.objects.get(id=50)
>>> b.publisher
<Publisher: Apress Publishing>
>>> b.publisher.website
u'http://www.apress.com/'

With ForeignKey fields, it works the other way, too, but it’s slightly different due to the non-symmetrical nature of the relationship. To get a list of books for a given publisher, use publisher.book_set.all(), like this:

>>> p = Publisher.objects.get(name='Apress Publishing')
>>> p.book_set.all()
[<Book: The Django Book>, <Book: Dive Into Python>, ...]

Behind the scenes, book_set is just a QuerySet (as covered in Chapter 5), and it can be filtered and sliced like any other QuerySet. For example:

>>> p = Publisher.objects.get(name='Apress Publishing')
>>> p.book_set.filter(title__icontains='django')
[<Book: The Django Book>, <Book: Pro Django>]

The attribute name book_set is generated by appending the lower case model name to _set.

Accessing Many-to-Many Values

Many-to-many values work like foreign-key values, except we deal with QuerySet values instead of model instances. For example, here’s how to view the authors for a book:

>>> b = Book.objects.get(id=50)
>>> b.authors.all()
[<Author: Adrian Holovaty>, <Author: Jacob Kaplan-Moss>]
>>> b.authors.filter(first_name='Adrian')
[<Author: Adrian Holovaty>]
>>> b.authors.filter(first_name='Adam')
[]

It works in reverse, too. To view all of the books for an author, use author.book_set, like this:

>>> a = Author.objects.get(first_name='Adrian', last_name='Holovaty')
>>> a.book_set.all()
[<Book: The Django Book>, <Book: Adrian's Other Book>]

Here, as with ForeignKey fields, the attribute name book_set is generated by appending the lower case model name to _set.

Adding Fields

When adding a field to a table/model in a production setting, the trick is to take advantage of the fact that Django doesn’t care if a table contains columns that aren’t defined in the model. The strategy is to add the column in the database, and then update the Django model to include the new field.

However, there’s a bit of a chicken-and-egg problem here, because in order to know how the new database column should be expressed in SQL, you need to look at the output of Django’s manage.py sqlall command, which requires that the field exist in the model. (Note that you’re not required to create your column with exactly the same SQL that Django would, but it’s a good idea to do so, just to be sure everything’s in sync.)

The solution to the chicken-and-egg problem is to use a development environment instead of making the changes on a production server. (You are using a testing/development environment, right?) Here are the detailed steps to take.

First, take these steps in the development environment (i.e., not on the production server):

1. Add the field to your model.
2. Run manage.py sqlall [yourapp] to see the new CREATE TABLE statement for the model. Note the column definition for the new field.
3. Start your database’s interactive shell (e.g., psql or mysql, or you can use manage.py dbshell). Execute an ALTER TABLE statement that adds your new column.
4. Launch the Python interactive shell with manage.py shell and verify that the new field was added properly by importing the model and selecting from the table (e.g., MyModel.objects.all()[:5]). If you updated the database correctly, the statement should work without errors.

Then on the production server perform these steps:

1. Start your database’s interactive shell.
2. Execute the ALTER TABLE statement you used in step 3 of the development environment steps.
3. Add the field to your model. If you’re using source-code revision control and you checked in your change in development environment step 1, now is the time to update the code (e.g., svn update, with Subversion) on the production server.
4. Restart the Web server for the code changes to take effect.

For example, let’s walk through what we’d do if we added a num_pages field to the Book model from Chapter 5. First, we’d alter the model in our development environment to look like this:

class Book(models.Model):
    title = models.CharField(max_length=100)
    authors = models.ManyToManyField(Author)
    publisher = models.ForeignKey(Publisher)
    publication_date = models.DateField()
    num_pages = models.IntegerField(blank=True, null=True)

    def __unicode__(self):
        return self.title

(Note: Read the section “Making Fields Optional” in Chapter 6, plus the sidebar “Adding NOT NULL Columns” below for important details on why we included blank=True and null=True.)

Then we’d run the command manage.py sqlall books to see the CREATE TABLE statement. Depending on your database backend, it would look something like this:

CREATE TABLE "books_book" (
    "id" serial NOT NULL PRIMARY KEY,
    "title" varchar(100) NOT NULL,
    "publisher_id" integer NOT NULL REFERENCES "books_publisher" ("id"),
    "publication_date" date NOT NULL,
    "num_pages" integer NULL
);

The new column is represented like this:

"num_pages" integer NULL

Next, we’d start the database’s interactive shell for our development database by typing psql (for PostgreSQL), and we’d execute the following statements:

ALTER TABLE books_book ADD COLUMN num_pages integer;

BEGIN;
ALTER TABLE books_book ADD COLUMN num_pages integer;
UPDATE books_book SET num_pages=0;
ALTER TABLE books_book ALTER COLUMN num_pages SET NOT NULL;
COMMIT;

After the ALTER TABLE statement, we’d verify that the change worked properly by starting the Python shell and running this code:

>>> from mysite.books.models import Book
>>> Book.objects.all()[:5]

If that code didn’t cause errors, we’d switch to our production server and execute the ALTER TABLE statement on the production database. Then, we’d update the model in the production environment and restart the Web server.

Removing Fields

Removing a field from a model is a lot easier than adding one. To remove a field, just follow these steps:

1. Remove the field from your model and restart the Web server.

2. Remove the column from your database, using a command like this:

   ALTER TABLE books_book DROP COLUMN num_pages;

Make sure to do it in this order. If you remove the column from your database first, Django will immediately begin raising errors.

Removing Many-to-Many Fields

Because many-to-many fields are different than normal fields, the removal process is different:

1. Remove the ManyToManyField from your model and restart the Web server.

2. Remove the many-to-many table from your database, using a command like this:

   DROP TABLE books_book_authors;

As in the previous section, make sure to do it in this order.

Removing Models

Removing a model entirely is as easy as removing a field. To remove a model, just follow these steps:

1. Remove the model from your models.py file and restart the Web server.

2. Remove the table from your database, using a command like this:

   DROP TABLE books_book;

   Note that you might need to remove any dependent tables from your database first – e.g., any tables that have foreign keys to books_book.

As in the previous sections, make sure to do it in this order.

Adding Extra Manager Methods

Adding extra manager methods is the preferred way to add “table-level” functionality to your models. (For “row-level” functionality – i.e., functions that act on a single instance of a model object – use model methods, which are explained later in this chapter.)

For example, let’s give our Book model a manager method title_count() that takes a keyword and returns the number of books that have a title containing that keyword. (This example is slightly contrived, but it demonstrates how managers work.)

# models.py

from django.db import models

# ... Author and Publisher models here ...

class BookManager(models.Manager):
    def title_count(self, keyword):
        return self.filter(title__icontains=keyword).count()

class Book(models.Model):
    title = models.CharField(max_length=100)
    authors = models.ManyToManyField(Author)
    publisher = models.ForeignKey(Publisher)
    publication_date = models.DateField()
    num_pages = models.IntegerField(blank=True, null=True)
    objects = BookManager()

    def __unicode__(self):
        return self.title

With this manager in place, we can now do this:

>>> Book.objects.title_count('django')
4
>>> Book.objects.title_count('python')
18

Here are some notes about the code:

• We’ve created a BookManager class that extends django.db.models.Manager. This has a single method, title_count(), which does the calculation. Note that the method uses self.filter(), where self refers to the manager itself.
• We’ve assigned BookManager() to the objects attribute on the model. This has the effect of replacing the “default” manager for the model, which is called objects and is automatically created if you don’t specify a custom manager. We call it objects rather than something else, so as to be consistent with automatically created managers.

Why would we want to add a method such as title_count()? To encapsulate commonly executed queries so that we don’t have to duplicate code.

Modifying Initial Manager QuerySets

A manager’s base QuerySet returns all objects in the system. For example, Book.objects.all() returns all books in the book database.

You can override a manager’s base QuerySet by overriding the Manager.get_query_set() method. get_query_set() should return a QuerySet with the properties you require.

For example, the following model has two managers – one that returns all objects, and one that returns only the books by Roald Dahl.

from django.db import models

# First, define the Manager subclass.
class DahlBookManager(models.Manager):
    def get_query_set(self):
        return super(DahlBookManager, self).get_query_set().filter(author='Roald Dahl')

# Then hook it into the Book model explicitly.
class Book(models.Model):
    title = models.CharField(max_length=100)
    author = models.CharField(max_length=50)
    # ...

    objects = models.Manager() # The default manager.
    dahl_objects = DahlBookManager() # The Dahl-specific manager.

With this sample model, Book.objects.all() will return all books in the database, but Book.dahl_objects.all() will only return the ones written by Roald Dahl. Note that we explicitly set objects to a vanilla Manager instance, because if we hadn’t, the only available manager would be dahl_objects.

Of course, because get_query_set() returns a QuerySet object, you can use filter(), exclude() and all the other QuerySet methods on it. So these statements are all legal:

Book.dahl_objects.all()
Book.dahl_objects.filter(title='Matilda')
Book.dahl_objects.count()

This example also pointed out another interesting technique: using multiple managers on the same model. You can attach as many Manager() instances to a model as you’d like. This is an easy way to define common “filters” for your models.

For example:

class MaleManager(models.Manager):
    def get_query_set(self):
        return super(MaleManager, self).get_query_set().filter(sex='M')

class FemaleManager(models.Manager):
    def get_query_set(self):
        return super(FemaleManager, self).get_query_set().filter(sex='F')

class Person(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    sex = models.CharField(max_length=1, choices=(('M', 'Male'), ('F', 'Female')))
    people = models.Manager()
    men = MaleManager()
    women = FemaleManager()

This example allows you to request Person.men.all(), Person.women.all(), and Person.people.all(), yielding predictable results.

If you use custom Manager objects, take note that the first Manager Django encounters (in the order in which they’re defined in the model) has a special status. Django interprets this first Manager defined in a class as the “default” Manager, and several parts of Django (though not the admin application) will use that Manager exclusively for that model. As a result, it’s often a good idea to be careful in your choice of default manager, in order to avoid a situation where overriding of get_query_set() results in an inability to retrieve objects you’d like to work with.