Step 3: Creating The Database¶
Flaskr is a database powered application as outlined earlier, and more precisely, an application powered by a relational database system. Such systems need a schema that tells them how to store that information. So before starting the server for the first time it’s important to create that schema.
Such a schema can be created by piping the schema.sql file into the sqlite3 command as follows:
sqlite3 /tmp/flaskr.db < schema.sql
The downside of this is that it requires the sqlite3 command to be installed which is not necessarily the case on every system. Also one has to provide the path to the database there which leaves some place for errors. It’s a good idea to add a function that initializes the database for you to the application.
If you want to do that, you first have to import the
contextlib.closing() function from the contextlib package.
Accordingly, add the following lines to your existing imports in flaskr.py:
from contextlib import closing
Next we can create a function called init_db that initializes the database. For this we can use the connect_db function we defined earlier. Just add that function below the connect_db function in flaskr.py:
def init_db(): with closing(connect_db()) as db: with app.open_resource('schema.sql', mode='r') as f: db.cursor().executescript(f.read()) db.commit()
closing() helper function allows us to keep a
connection open for the duration of the with block. The
open_resource() method of the application object
supports that functionality out of the box, so it can be used in the
with block directly. This function opens a file from the resource
location (your flaskr folder) and allows you to read from it. We are
using this here to execute a script on the database connection.
When we connect to a database we get a connection object (here called db) that can give us a cursor. On that cursor there is a method to execute a complete script. Finally we only have to commit the changes. SQLite 3 and other transactional databases will not commit unless you explicitly tell it to.
Now it is possible to create a database by starting up a Python shell and importing and calling that function:
>>> from flaskr import init_db >>> init_db()
If you get an exception later that a table cannot be found check that you did call the init_db function and that your table names are correct (singular vs. plural for example).
Continue with Step 4: Request Database Connections