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63 lines
2.7 KiB
63 lines
2.7 KiB
Step 3: Creating The Database |
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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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