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69 lines
2.9 KiB
69 lines
2.9 KiB
.. _tutorial-dbinit: |
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Step 4: Creating The Database |
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============================= |
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As outlined earlier, Flaskr is a database powered application, and more |
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precisely, it is 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. This also |
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require that we provide the path to the database which can introduce |
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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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To do this we can create a function called `init_db` that initializes the |
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database. Let me show you the code first. Just add this function below |
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the `connect_db` function in `flaskr.py`:: |
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def init_db(): |
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with app.app_context(): |
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db = get_db() |
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with app.open_resource('schema.sql', mode='r') as f: |
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db.cursor().executescript(f.read()) |
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db.commit() |
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So what's happening here? Remember how we learned last chapter that the |
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application context is created every time a request comes in? Here we |
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don't have a request yet, so we need to create the application context by |
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hand. Without an application context the :data:`~flask.g` object does not |
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know yet to which application it becomes as there could be more than one! |
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The ``with app.app_context()`` statement establishes the application |
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context for us. In the body of the with statement the :data:`~flask.g` |
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object will be associated with ``app``. At the end of the with statement |
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the association is released and all teardown functions are executed. This |
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means that our database connection is disconnected after the commit. |
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The :func:`~flask.Flask.open_resource` method of the application object |
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is a convenient helper function that will open a resource that the |
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application provides. 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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The connection object provided by SQLite can give us a cursor object. |
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On that cursor there is a method to execute a complete script. Finally we |
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only have to commit the changes. SQLite 3 and other transactional |
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databases will not commit unless you 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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Continue with :ref:`tutorial-views`
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