flask/docs/tutorial/database.rst

.. currentmodule:: flask

Define and Access the Database
==============================

The application will use a `SQLite`_ database to store users and posts.
Python comes with built-in support for SQLite in the :mod:`sqlite3`
module.

SQLite is convenient because it doesn't require setting up a separate
database server and is built-in to Python. However, if concurrent
requests try to write to the database at the same time, they will slow
down as each write happens sequentially. Small applications won't notice
this. Once you become big, you may want to switch to a different
database.

The tutorial doesn't go into detail about SQL. If you are not familiar
with it, the SQLite docs describe the `language`_.

.. _SQLite: https://sqlite.org/about.html
.. _language: https://sqlite.org/lang.html


Connect to the Database
-----------------------

The first thing to do when working with a SQLite database (and most
other Python database libraries) is to create a connection to it. Any
queries and operations are performed using the connection, which is
closed after the work is finished.

In web applications this connection is typically tied to the request. It
is created at some point when handling a request, and closed before the
response is sent.

.. code-block:: python
    :caption: ``flaskr/db.py``

    import sqlite3

    import click
    from flask import current_app, g


    def get_db():
        if 'db' not in g:
            g.db = sqlite3.connect(
                current_app.config['DATABASE'],
                detect_types=sqlite3.PARSE_DECLTYPES
            )
            g.db.row_factory = sqlite3.Row

        return g.db


    def close_db(e=None):
        db = g.pop('db', None)

        if db is not None:
            db.close()

:data:`g` is a special object that is unique for each request. It is
used to store data that might be accessed by multiple functions during
the request. The connection is stored and reused instead of creating a
new connection if ``get_db`` is called a second time in the same
request.

:data:`current_app` is another special object that points to the Flask
application handling the request. Since you used an application factory,
there is no application object when writing the rest of your code.
``get_db`` will be called when the application has been created and is
handling a request, so :data:`current_app` can be used.

:func:`sqlite3.connect` establishes a connection to the file pointed at
by the ``DATABASE`` configuration key. This file doesn't have to exist
yet, and won't until you initialize the database later.

:class:`sqlite3.Row` tells the connection to return rows that behave
like dicts. This allows accessing the columns by name.

``close_db`` checks if a connection was created by checking if ``g.db``
was set. If the connection exists, it is closed. Further down you will
tell your application about the ``close_db`` function in the application
factory so that it is called after each request.


Create the Tables
-----------------

In SQLite, data is stored in *tables* and *columns*. These need to be
created before you can store and retrieve data. Flaskr will store users
in the ``user`` table, and posts in the ``post`` table. Create a file
with the SQL commands needed to create empty tables:

.. code-block:: sql
    :caption: ``flaskr/schema.sql``

    DROP TABLE IF EXISTS user;
    DROP TABLE IF EXISTS post;

    CREATE TABLE user (
      id INTEGER PRIMARY KEY AUTOINCREMENT,
      username TEXT UNIQUE NOT NULL,
      password TEXT NOT NULL
    );

    CREATE TABLE post (
      id INTEGER PRIMARY KEY AUTOINCREMENT,
      author_id INTEGER NOT NULL,
      created TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
      title TEXT NOT NULL,
      body TEXT NOT NULL,
      FOREIGN KEY (author_id) REFERENCES user (id)
    );

Add the Python functions that will run these SQL commands to the
``db.py`` file:

.. code-block:: python
    :caption: ``flaskr/db.py``

    def init_db():
        db = get_db()

        with current_app.open_resource('schema.sql') as f:
            db.executescript(f.read().decode('utf8'))


    @click.command('init-db')
    def init_db_command():
        """Clear the existing data and create new tables."""
        init_db()
        click.echo('Initialized the database.')

:meth:`open_resource() <Flask.open_resource>` opens a file relative to
the ``flaskr`` package, which is useful since you won't necessarily know
where that location is when deploying the application later. ``get_db``
returns a database connection, which is used to execute the commands
read from the file.

:func:`click.command` defines a command line command called ``init-db``
that calls the ``init_db`` function and shows a success message to the
user. You can read :doc:`/cli` to learn more about writing commands.


Register with the Application
-----------------------------

The ``close_db`` and ``init_db_command`` functions need to be registered
with the application instance; otherwise, they won't be used by the
application. However, since you're using a factory function, that
instance isn't available when writing the functions. Instead, write a
function that takes an application and does the registration.

.. code-block:: python
    :caption: ``flaskr/db.py``

    def init_app(app):
        app.teardown_appcontext(close_db)
        app.cli.add_command(init_db_command)

:meth:`app.teardown_appcontext() <Flask.teardown_appcontext>` tells
Flask to call that function when cleaning up after returning the
response.

:meth:`app.cli.add_command() <click.Group.add_command>` adds a new
command that can be called with the ``flask`` command.

Import and call this function from the factory. Place the new code at
the end of the factory function before returning the app.

.. code-block:: python
    :caption: ``flaskr/__init__.py``

    def create_app():
        app = ...
        # existing code omitted

        from . import db
        db.init_app(app)

        return app


Initialize the Database File
----------------------------

Now that ``init-db`` has been registered with the app, it can be called
using the ``flask`` command, similar to the ``run`` command from the
previous page.

.. note::

    If you're still running the server from the previous page, you can
    either stop the server, or run this command in a new terminal. If
    you use a new terminal, remember to change to your project directory
    and activate the env as described in :doc:`/installation`.

Run the ``init-db`` command:

.. code-block:: none

    $ flask --app flaskr init-db
    Initialized the database.

There will now be a ``flaskr.sqlite`` file in the ``instance`` folder in
your project.

Continue to :doc:`views`.