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Step 3: Creating The Database
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=============================
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Flaskr is a database powered application as outlined earlier, and more
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precisely, an application powered by a relational database system. Such
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systems need a schema that tells them how to store that information. So
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before starting the server for the first time it's important to create
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that schema.
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Such a schema can be created by piping the `schema.sql` file into the
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`sqlite3` command as follows::
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sqlite3 /tmp/flaskr.db < schema.sql
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The downside of this is that it requires the sqlite3 command to be
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installed which is not necessarily the case on every system. Also one has
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to provide the path to the database there which leaves some place for
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errors. It's a good idea to add a function that initializes the database
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for you to the application.
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If you want to do that, you first have to import the
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:func:`contextlib.closing` function from the contextlib package. If you
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want to use Python 2.5 it's also necessary to enable the `with` statement
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first (`__future__` imports must be the very first import)::
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from __future__ import with_statement
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from contextlib import closing
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Next we can create a function called `init_db` that initializes the
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database. For this we can use the `connect_db` function we defined
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earlier. Just add that function below the `connect_db` function::
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def init_db():
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with closing(connect_db()) as db:
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with app.open_resource('schema.sql') as f:
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db.cursor().executescript(f.read())
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db.commit()
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The :func:`~contextlib.closing` helper function allows us to keep a
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connection open for the duration of the `with` block. The
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:func:`~flask.Flask.open_resource` method of the application object
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supports that functionality out of the box, so it can be used in the
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`with` block directly. This function opens a file from the resource
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location (your `flaskr` folder) and allows you to read from it. We are
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using this here to execute a script on the database connection.
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When we connect to a database we get a connection object (here called
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`db`) that can give us a cursor. On that cursor there is a method to
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execute a complete script. Finally we only have to commit the changes.
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SQLite 3 and other transactional databases will not commit unless you
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explicitly tell it to.
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Now it is possible to create a database by starting up a Python shell and
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importing and calling that function::
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>>> from flaskr import init_db
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>>> init_db()
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.. admonition:: Troubleshooting
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If you get an exception later that a table cannot be found check that
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you did call the `init_db` function and that your table names are
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correct (singular vs. plural for example).
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