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689 lines
26 KiB
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689 lines
26 KiB
Plaintext
=====================================
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Writing your first Django app, part 1
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=====================================
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Let's learn by example.
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Throughout this tutorial, we'll walk you through the creation of a basic
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poll application.
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It'll consist of two parts:
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* A public site that lets people view polls and vote in them.
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* An admin site that lets you add, change and delete polls.
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We'll assume you have :doc:`Django installed </intro/install>` already. You can
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tell Django is installed by running the Python interactive interpreter and
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typing ``import django``. If that command runs successfully, with no errors,
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Django is installed.
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.. admonition:: Where to get help:
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If you're having trouble going through this tutorial, please post a message
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to `django-users`__ or drop by `#django on irc.freenode.net`__ to chat
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with other Django users who might be able to help.
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__ http://groups.google.com/group/django-users
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__ irc://irc.freenode.net/django
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Creating a project
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==================
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If this is your first time using Django, you'll have to take care of some
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initial setup. Namely, you'll need to auto-generate some code that establishes a
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Django :term:`project` -- a collection of settings for an instance of Django,
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including database configuration, Django-specific options and
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application-specific settings.
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From the command line, ``cd`` into a directory where you'd like to store your
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code, then run the command ``django-admin.py startproject mysite``. This will
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create a ``mysite`` directory in your current directory.
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.. admonition:: Script name may differ in distribution packages
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If you installed Django using a Linux distribution's package manager
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(e.g. apt-get or yum) ``django-admin.py`` may have been renamed to
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``django-admin``. You may continue through this documentation by omitting
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``.py`` from each command.
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.. admonition:: Mac OS X permissions
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If you're using Mac OS X, you may see the message "permission denied" when
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you try to run ``django-admin.py startproject``. This is because, on
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Unix-based systems like OS X, a file must be marked as "executable" before it
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can be run as a program. To do this, open Terminal.app and navigate (using
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the ``cd`` command) to the directory where :doc:`django-admin.py
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</ref/django-admin>` is installed, then run the command
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``chmod +x django-admin.py``.
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.. note::
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You'll need to avoid naming projects after built-in Python or Django
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components. In particular, this means you should avoid using names like
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``django`` (which will conflict with Django itself) or ``test`` (which
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conflicts with a built-in Python package).
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:doc:`django-admin.py </ref/django-admin>` should be on your system path if you
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installed Django via ``python setup.py``. If it's not on your path, you can find
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it in ``site-packages/django/bin``, where ```site-packages``` is a directory
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within your Python installation. Consider symlinking to :doc:`django-admin.py
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</ref/django-admin>` from some place on your path, such as
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:file:`/usr/local/bin`.
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.. admonition:: Where should this code live?
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If your background is in PHP, you're probably used to putting code under the
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Web server's document root (in a place such as ``/var/www``). With Django,
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you don't do that. It's not a good idea to put any of this Python code
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within your Web server's document root, because it risks the possibility
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that people may be able to view your code over the Web. That's not good for
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security.
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Put your code in some directory **outside** of the document root, such as
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:file:`/home/mycode`.
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Let's look at what :djadmin:`startproject` created::
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mysite/
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__init__.py
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manage.py
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settings.py
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urls.py
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These files are:
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* :file:`__init__.py`: An empty file that tells Python that this directory
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should be considered a Python package. (Read `more about packages`_ in the
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official Python docs if you're a Python beginner.)
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* :file:`manage.py`: A command-line utility that lets you interact with this
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Django project in various ways. You can read all the details about
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:file:`manage.py` in :doc:`/ref/django-admin`.
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* :file:`settings.py`: Settings/configuration for this Django project.
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:doc:`/topics/settings` will tell you all about how settings work.
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* :file:`urls.py`: The URL declarations for this Django project; a "table of
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contents" of your Django-powered site. You can read more about URLs in
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:doc:`/topics/http/urls`.
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.. _more about packages: http://docs.python.org/tutorial/modules.html#packages
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The development server
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----------------------
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Let's verify this worked. Change into the :file:`mysite` directory, if you
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haven't already, and run the command ``python manage.py runserver``. You'll see
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the following output on the command line::
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Validating models...
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0 errors found.
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Django version 1.0, using settings 'mysite.settings'
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Development server is running at http://127.0.0.1:8000/
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Quit the server with CONTROL-C.
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You've started the Django development server, a lightweight Web server written
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purely in Python. We've included this with Django so you can develop things
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rapidly, without having to deal with configuring a production server -- such as
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Apache -- until you're ready for production.
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Now's a good time to note: DON'T use this server in anything resembling a
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production environment. It's intended only for use while developing. (We're in
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the business of making Web frameworks, not Web servers.)
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Now that the server's running, visit http://127.0.0.1:8000/ with your Web
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browser. You'll see a "Welcome to Django" page, in pleasant, light-blue pastel.
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It worked!
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.. admonition:: Changing the port
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By default, the :djadmin:`runserver` command starts the development server
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on the internal IP at port 8000.
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If you want to change the server's port, pass
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it as a command-line argument. For instance, this command starts the server
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on port 8080:
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.. code-block:: bash
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python manage.py runserver 8080
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If you want to change the server's IP, pass it along with the port. So to
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listen on all public IPs (useful if you want to show off your work on other
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computers), use:
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.. code-block:: bash
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python manage.py runserver 0.0.0.0:8000
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Full docs for the development server can be found in the
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:djadmin:`runserver` reference.
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Database setup
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--------------
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Now, edit :file:`settings.py`. It's a normal Python module with
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module-level variables representing Django settings. Change the
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following keys in the :setting:`DATABASES` ``'default'`` item to match
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your databases connection settings.
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* :setting:`ENGINE` -- Either
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``'django.db.backends.postgresql_psycopg2'``,
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``'django.db.backends.mysql'`` or
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``'django.db.backends.sqlite3'``. Other backends are
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:setting:`also available <ENGINE>`.
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* :setting:`NAME` -- The name of your database. If you're using
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SQLite, the database will be a file on your computer; in that
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case, :setting:`NAME` should be the full absolute path,
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including filename, of that file. If the file doesn't exist, it
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will automatically be created when you synchronize the database
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for the first time (see below).
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When specifying the path, always use forward slashes, even on
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Windows (e.g. ``C:/homes/user/mysite/sqlite3.db``).
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* :setting:`USER` -- Your database username (not used for SQLite).
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* :setting:`PASSWORD` -- Your database password (not used for
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SQLite).
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* :setting:`HOST` -- The host your database is on. Leave this as
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an empty string if your database server is on the same physical
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machine (not used for SQLite).
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If you're new to databases, we recommend simply using SQLite (by
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setting :setting:`ENGINE` to ``'django.db.backends.sqlite3'``). SQLite
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is included as part of Python 2.5 and later, so you won't need to
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install anything else.
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.. note::
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If you're using PostgreSQL or MySQL, make sure you've created a database by
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this point. Do that with "``CREATE DATABASE database_name;``" within your
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database's interactive prompt.
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If you're using SQLite, you don't need to create anything beforehand - the
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database file will be created automatically when it is needed.
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While you're editing :file:`settings.py`, take note of the
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:setting:`INSTALLED_APPS` setting towards the bottom of the file. That variable
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holds the names of all Django applications that are activated in this Django
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instance. Apps can be used in multiple projects, and you can package and
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distribute them for use by others in their projects.
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By default, :setting:`INSTALLED_APPS` contains the following apps, all of which
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come with Django:
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* :mod:`django.contrib.auth` -- An authentication system.
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* :mod:`django.contrib.contenttypes` -- A framework for content types.
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* :mod:`django.contrib.sessions` -- A session framework.
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* :mod:`django.contrib.sites` -- A framework for managing multiple sites
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with one Django installation.
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These applications are included by default as a convenience for the common case.
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Each of these applications makes use of at least one database table, though,
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so we need to create the tables in the database before we can use them. To do
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that, run the following command:
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.. code-block:: bash
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python manage.py syncdb
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The :djadmin:`syncdb` command looks at the :setting:`INSTALLED_APPS` setting and
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creates any necessary database tables according to the database settings in your
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:file:`settings.py` file. You'll see a message for each database table it
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creates, and you'll get a prompt asking you if you'd like to create a superuser
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account for the authentication system. Go ahead and do that.
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If you're interested, run the command-line client for your database and type
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``\dt`` (PostgreSQL), ``SHOW TABLES;`` (MySQL), or ``.schema`` (SQLite) to
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display the tables Django created.
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.. admonition:: For the minimalists
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Like we said above, the default applications are included for the common
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case, but not everybody needs them. If you don't need any or all of them,
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feel free to comment-out or delete the appropriate line(s) from
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:setting:`INSTALLED_APPS` before running :djadmin:`syncdb`. The
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:djadmin:`syncdb` command will only create tables for apps in
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:setting:`INSTALLED_APPS`.
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.. _creating-models:
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Creating models
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===============
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Now that your environment -- a "project" -- is set up, you're set to start
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doing work.
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Each application you write in Django consists of a Python package, somewhere
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on your `Python path`_, that follows a certain convention. Django comes with a
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utility that automatically generates the basic directory structure of an app,
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so you can focus on writing code rather than creating directories.
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.. admonition:: Projects vs. apps
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What's the difference between a project and an app? An app is a Web
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application that does something -- e.g., a Weblog system, a database of
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public records or a simple poll app. A project is a collection of
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configuration and apps for a particular Web site. A project can contain
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multiple apps. An app can be in multiple projects.
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Your apps can live anywhere on your `Python path`_. In this tutorial, we'll
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create our poll app in the :file:`mysite` directory for simplicity.
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To create your app, make sure you're in the :file:`mysite` directory and type
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this command:
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.. code-block:: bash
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python manage.py startapp polls
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That'll create a directory :file:`polls`, which is laid out like this::
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polls/
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__init__.py
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models.py
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tests.py
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views.py
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This directory structure will house the poll application.
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The first step in writing a database Web app in Django is to define your models
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-- essentially, your database layout, with additional metadata.
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.. admonition:: Philosophy
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A model is the single, definitive source of data about your data. It contains
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the essential fields and behaviors of the data you're storing. Django follows
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the :ref:`DRY Principle <dry>`. The goal is to define your data model in one
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place and automatically derive things from it.
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In our simple poll app, we'll create two models: polls and choices. A poll has
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a question and a publication date. A choice has two fields: the text of the
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choice and a vote tally. Each choice is associated with a poll.
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These concepts are represented by simple Python classes. Edit the
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:file:`polls/models.py` file so it looks like this::
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from django.db import models
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class Poll(models.Model):
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question = models.CharField(max_length=200)
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pub_date = models.DateTimeField('date published')
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class Choice(models.Model):
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poll = models.ForeignKey(Poll)
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choice = models.CharField(max_length=200)
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votes = models.IntegerField()
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The code is straightforward. Each model is represented by a class that
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subclasses :class:`django.db.models.Model`. Each model has a number of class
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variables, each of which represents a database field in the model.
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Each field is represented by an instance of a :class:`~django.db.models.Field`
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class -- e.g., :class:`~django.db.models.CharField` for character fields and
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:class:`~django.db.models.DateTimeField` for datetimes. This tells Django what
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type of data each field holds.
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The name of each :class:`~django.db.models.Field` instance (e.g. ``question`` or
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``pub_date`` ) is the field's name, in machine-friendly format. You'll use this
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value in your Python code, and your database will use it as the column name.
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You can use an optional first positional argument to a
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:class:`~django.db.models.Field` to designate a human-readable name. That's used
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in a couple of introspective parts of Django, and it doubles as documentation.
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If this field isn't provided, Django will use the machine-readable name. In this
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example, we've only defined a human-readable name for ``Poll.pub_date``. For all
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other fields in this model, the field's machine-readable name will suffice as
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its human-readable name.
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Some :class:`~django.db.models.Field` classes have required elements.
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:class:`~django.db.models.CharField`, for example, requires that you give it a
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:attr:`~django.db.models.Field.max_length`. That's used not only in the database
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schema, but in validation, as we'll soon see.
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Finally, note a relationship is defined, using
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:class:`~django.db.models.ForeignKey`. That tells Django each Choice is related
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to a single Poll. Django supports all the common database relationships:
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many-to-ones, many-to-manys and one-to-ones.
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.. _`Python path`: http://docs.python.org/tutorial/modules.html#the-module-search-path
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Activating models
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=================
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That small bit of model code gives Django a lot of information. With it, Django
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is able to:
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* Create a database schema (``CREATE TABLE`` statements) for this app.
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* Create a Python database-access API for accessing Poll and Choice objects.
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But first we need to tell our project that the ``polls`` app is installed.
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.. admonition:: Philosophy
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Django apps are "pluggable": You can use an app in multiple projects, and
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you can distribute apps, because they don't have to be tied to a given
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Django installation.
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Edit the :file:`settings.py` file again, and change the
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:setting:`INSTALLED_APPS` setting to include the string ``'polls'``. So
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it'll look like this::
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INSTALLED_APPS = (
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'django.contrib.auth',
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'django.contrib.contenttypes',
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'django.contrib.sessions',
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'django.contrib.sites',
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'polls'
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)
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Now Django knows to include the ``polls`` app. Let's run another
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command:
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.. code-block:: bash
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python manage.py sql polls
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You should see something similar to the following (the ``CREATE TABLE`` SQL
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statements for the polls app):
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.. code-block:: sql
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BEGIN;
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CREATE TABLE "polls_poll" (
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"id" serial NOT NULL PRIMARY KEY,
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"question" varchar(200) NOT NULL,
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"pub_date" timestamp with time zone NOT NULL
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);
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CREATE TABLE "polls_choice" (
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"id" serial NOT NULL PRIMARY KEY,
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"poll_id" integer NOT NULL REFERENCES "polls_poll" ("id"),
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"choice" varchar(200) NOT NULL,
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"votes" integer NOT NULL
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);
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COMMIT;
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Note the following:
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* The exact output will vary depending on the database you are using.
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* Table names are automatically generated by combining the name of the app
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(``polls``) and the lowercase name of the model -- ``poll`` and
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``choice``. (You can override this behavior.)
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* Primary keys (IDs) are added automatically. (You can override this, too.)
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* By convention, Django appends ``"_id"`` to the foreign key field name.
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Yes, you can override this, as well.
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* The foreign key relationship is made explicit by a ``REFERENCES``
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statement.
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* It's tailored to the database you're using, so database-specific field
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types such as ``auto_increment`` (MySQL), ``serial`` (PostgreSQL), or
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``integer primary key`` (SQLite) are handled for you automatically. Same
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goes for quoting of field names -- e.g., using double quotes or single
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quotes. The author of this tutorial runs PostgreSQL, so the example
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output is in PostgreSQL syntax.
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* The :djadmin:`sql` command doesn't actually run the SQL in your database -
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it just prints it to the screen so that you can see what SQL Django thinks
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is required. If you wanted to, you could copy and paste this SQL into your
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database prompt. However, as we will see shortly, Django provides an
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easier way of committing the SQL to the database.
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If you're interested, also run the following commands:
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* :djadmin:`python manage.py validate <validate>` -- Checks for any errors
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in the construction of your models.
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* :djadmin:`python manage.py sqlcustom polls <sqlcustom>` -- Outputs any
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:ref:`custom SQL statements <initial-sql>` (such as table modifications or
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constraints) that are defined for the application.
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* :djadmin:`python manage.py sqlclear polls <sqlclear>` -- Outputs the
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necessary ``DROP TABLE`` statements for this app, according to which
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tables already exist in your database (if any).
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* :djadmin:`python manage.py sqlindexes polls <sqlindexes>` -- Outputs the
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``CREATE INDEX`` statements for this app.
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* :djadmin:`python manage.py sqlall polls <sqlall>` -- A combination of all
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the SQL from the :djadmin:`sql`, :djadmin:`sqlcustom`, and
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:djadmin:`sqlindexes` commands.
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Looking at the output of those commands can help you understand what's actually
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happening under the hood.
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Now, run :djadmin:`syncdb` again to create those model tables in your database:
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.. code-block:: bash
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python manage.py syncdb
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The :djadmin:`syncdb` command runs the sql from 'sqlall' on your database for
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all apps in :setting:`INSTALLED_APPS` that don't already exist in your database.
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This creates all the tables, initial data and indexes for any apps you have
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added to your project since the last time you ran syncdb. :djadmin:`syncdb` can
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be called as often as you like, and it will only ever create the tables that
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don't exist.
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Read the :doc:`django-admin.py documentation </ref/django-admin>` for full
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information on what the ``manage.py`` utility can do.
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Playing with the API
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====================
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Now, let's hop into the interactive Python shell and play around with the free
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API Django gives you. To invoke the Python shell, use this command:
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.. code-block:: bash
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python manage.py shell
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We're using this instead of simply typing "python", because ``manage.py`` sets
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up the project's environment for you. "Setting up the environment" involves two
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things:
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* Putting ``polls`` on ``sys.path``. For flexibility, several pieces of
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Django refer to projects in Python dotted-path notation (e.g.
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``'polls.models'``). In order for this to work, the ``polls``
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package has to be on ``sys.path``.
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We've already seen one example of this: the :setting:`INSTALLED_APPS`
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setting is a list of packages in dotted-path notation.
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* Setting the ``DJANGO_SETTINGS_MODULE`` environment variable, which gives
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Django the path to your ``settings.py`` file.
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.. admonition:: Bypassing manage.py
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If you'd rather not use ``manage.py``, no problem. Just make sure ``mysite``
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and ``polls`` are at the root level on the Python path (i.e., ``import mysite``
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and ``import polls`` work) and set the ``DJANGO_SETTINGS_MODULE`` environment
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variable to ``mysite.settings``.
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For more information on all of this, see the :doc:`django-admin.py
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documentation </ref/django-admin>`.
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Once you're in the shell, explore the :doc:`database API </topics/db/queries>`::
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>>> from polls.models import Poll, Choice # Import the model classes we just wrote.
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# No polls are in the system yet.
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>>> Poll.objects.all()
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[]
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# Create a new Poll.
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>>> import datetime
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>>> p = Poll(question="What's up?", pub_date=datetime.datetime.now())
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# Save the object into the database. You have to call save() explicitly.
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>>> p.save()
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# Now it has an ID. Note that this might say "1L" instead of "1", depending
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# on which database you're using. That's no biggie; it just means your
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# database backend prefers to return integers as Python long integer
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# objects.
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>>> p.id
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1
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# Access database columns via Python attributes.
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>>> p.question
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"What's up?"
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>>> p.pub_date
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datetime.datetime(2007, 7, 15, 12, 00, 53)
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# Change values by changing the attributes, then calling save().
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>>> p.pub_date = datetime.datetime(2007, 4, 1, 0, 0)
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>>> p.save()
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# objects.all() displays all the polls in the database.
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>>> Poll.objects.all()
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[<Poll: Poll object>]
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Wait a minute. ``<Poll: Poll object>`` is, utterly, an unhelpful representation
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of this object. Let's fix that by editing the polls model (in the
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``polls/models.py`` file) and adding a
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:meth:`~django.db.models.Model.__unicode__` method to both ``Poll`` and
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``Choice``::
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class Poll(models.Model):
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# ...
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def __unicode__(self):
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return self.question
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class Choice(models.Model):
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# ...
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def __unicode__(self):
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return self.choice
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It's important to add :meth:`~django.db.models.Model.__unicode__` methods to
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your models, not only for your own sanity when dealing with the interactive
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prompt, but also because objects' representations are used throughout Django's
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automatically-generated admin.
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.. admonition:: Why :meth:`~django.db.models.Model.__unicode__` and not
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:meth:`~django.db.models.Model.__str__`?
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If you're familiar with Python, you might be in the habit of adding
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:meth:`~django.db.models.Model.__str__` methods to your classes, not
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:meth:`~django.db.models.Model.__unicode__` methods. We use
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:meth:`~django.db.models.Model.__unicode__` here because Django models deal
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with Unicode by default. All data stored in your database is converted to
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Unicode when it's returned.
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Django models have a default :meth:`~django.db.models.Model.__str__` method
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that calls :meth:`~django.db.models.Model.__unicode__` and converts the
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result to a UTF-8 bytestring. This means that ``unicode(p)`` will return a
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Unicode string, and ``str(p)`` will return a normal string, with characters
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encoded as UTF-8.
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If all of this is jibberish to you, just remember to add
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:meth:`~django.db.models.Model.__unicode__` methods to your models. With any
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luck, things should Just Work for you.
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Note these are normal Python methods. Let's add a custom method, just for
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demonstration::
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import datetime
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# ...
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class Poll(models.Model):
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# ...
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def was_published_today(self):
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return self.pub_date.date() == datetime.date.today()
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Note the addition of ``import datetime`` to reference Python's standard
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``datetime`` module.
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Save these changes and start a new Python interactive shell by running
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``python manage.py shell`` again::
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>>> from polls.models import Poll, Choice
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# Make sure our __unicode__() addition worked.
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>>> Poll.objects.all()
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[<Poll: What's up?>]
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# Django provides a rich database lookup API that's entirely driven by
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# keyword arguments.
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>>> Poll.objects.filter(id=1)
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[<Poll: What's up?>]
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>>> Poll.objects.filter(question__startswith='What')
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[<Poll: What's up?>]
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# Get the poll whose year is 2007.
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>>> Poll.objects.get(pub_date__year=2007)
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<Poll: What's up?>
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>>> Poll.objects.get(id=2)
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Traceback (most recent call last):
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...
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DoesNotExist: Poll matching query does not exist.
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# Lookup by a primary key is the most common case, so Django provides a
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# shortcut for primary-key exact lookups.
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# The following is identical to Poll.objects.get(id=1).
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>>> Poll.objects.get(pk=1)
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<Poll: What's up?>
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# Make sure our custom method worked.
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>>> p = Poll.objects.get(pk=1)
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>>> p.was_published_today()
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False
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# Give the Poll a couple of Choices. The create call constructs a new
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# choice object, does the INSERT statement, adds the choice to the set
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# of available choices and returns the new Choice object. Django creates
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# a set to hold the "other side" of a ForeignKey relation
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# (e.g. a poll's choices) which can be accessed via the API.
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>>> p = Poll.objects.get(pk=1)
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# Display any choices from the related object set -- none so far.
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>>> p.choice_set.all()
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[]
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# Create three choices.
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>>> p.choice_set.create(choice='Not much', votes=0)
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<Choice: Not much>
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>>> p.choice_set.create(choice='The sky', votes=0)
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<Choice: The sky>
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>>> c = p.choice_set.create(choice='Just hacking again', votes=0)
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# Choice objects have API access to their related Poll objects.
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>>> c.poll
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<Poll: What's up?>
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# And vice versa: Poll objects get access to Choice objects.
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>>> p.choice_set.all()
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[<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]
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>>> p.choice_set.count()
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3
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# The API automatically follows relationships as far as you need.
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# Use double underscores to separate relationships.
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# This works as many levels deep as you want; there's no limit.
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# Find all Choices for any poll whose pub_date is in 2007.
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>>> Choice.objects.filter(poll__pub_date__year=2007)
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[<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]
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# Let's delete one of the choices. Use delete() for that.
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>>> c = p.choice_set.filter(choice__startswith='Just hacking')
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>>> c.delete()
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For more information on model relations, see :doc:`Accessing related objects
|
|
</ref/models/relations>`. For full details on the database API, see our
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|
:doc:`Database API reference </topics/db/queries>`.
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When you're comfortable with the API, read :doc:`part 2 of this tutorial
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|
</intro/tutorial02>` to get Django's automatic admin working.
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