19: Databases Using SQLAlchemy

Store and retrieve data using the SQLAlchemy ORM atop the SQLite database.

Background

Our Pyramid-based wiki application now needs database-backed storage of pages. This frequently means an SQL database. The Pyramid community strongly supports the SQLAlchemy project and its object-relational mapper (ORM) as a convenient, Pythonic way to interface to databases.

In this step we hook up SQLAlchemy to a SQLite database table, providing storage and retrieval for the wiki pages in the previous step.

Note

The Pyramid cookiecutter pyramid-cookiecutter-starter is really helpful for getting a SQLAlchemy project going, including generation of the console script. Since we want to see all the decisions, we will forgo convenience in this tutorial, and wire it up ourselves.

Objectives

• Store pages in SQLite by using SQLAlchemy models.

• Use SQLAlchemy queries to list/add/view/edit pages.

• Provide a database-initialize command by writing a Pyramid console script which can be run from the command line.

Steps

1. We are going to use the forms step as our starting point:

   cd ..; cp -r forms databases; cd databases
   

2. We need to add some dependencies in databases/setup.py as well as an entry point for the command-line script:

   from setuptools import setup
   
   # List of dependencies installed via `pip install -e .`
   # by virtue of the Setuptools `install_requires` value below.
   requires = [
       'deform',
       'pyramid',
       'pyramid_chameleon',
       'pyramid_tm',
       'sqlalchemy',
       'waitress',
       'zope.sqlalchemy',
   ]
   
   # List of dependencies installed via `pip install -e ".[dev]"`
   # by virtue of the Setuptools `extras_require` value in the Python
   # dictionary below.
   dev_requires = [
       'pyramid_debugtoolbar',
       'pytest',
       'webtest',
   ]
   
   setup(
       name='tutorial',
       install_requires=requires,
       extras_require={
           'dev': dev_requires,
       },
       entry_points={
           'paste.app_factory': [
               'main = tutorial:main'
           ],
           'console_scripts': [
               'initialize_tutorial_db = tutorial.initialize_db:main'
           ],
       },
   )
   

   Note

   We aren't yet doing $VENV/bin/pip install -e . because we need to write a script and update configuration first.

3. Our configuration file at databases/development.ini wires together some new pieces:

   [app:main]
   use = egg:tutorial
   pyramid.reload_templates = true
   pyramid.includes =
       pyramid_debugtoolbar
       pyramid_tm
   
   sqlalchemy.url = sqlite:///%(here)s/sqltutorial.sqlite
   
   [server:main]
   use = egg:waitress#main
   listen = localhost:6543
   
   # Begin logging configuration
   
   [loggers]
   keys = root, tutorial, sqlalchemy.engine.Engine
   
   [logger_tutorial]
   level = DEBUG
   handlers =
   qualname = tutorial
   
   [handlers]
   keys = console
   
   [formatters]
   keys = generic
   
   [logger_root]
   level = INFO
   handlers = console
   
   [logger_sqlalchemy.engine.Engine]
   level = INFO
   handlers =
   qualname = sqlalchemy.engine.Engine
   
   [handler_console]
   class = StreamHandler
   args = (sys.stderr,)
   level = NOTSET
   formatter = generic
   
   [formatter_generic]
   format = %(asctime)s %(levelname)-5.5s [%(name)s][%(threadName)s] %(message)s
   
   # End logging configuration
   

4. This engine configuration now needs to be read into the application through changes in databases/tutorial/__init__.py:

   from pyramid.config import Configurator
   
   from sqlalchemy import engine_from_config
   
   from .models import DBSession, Base
   
   def main(global_config, **settings):
       engine = engine_from_config(settings, 'sqlalchemy.')
       DBSession.configure(bind=engine)
       Base.metadata.bind = engine
   
       config = Configurator(settings=settings,
                             root_factory='tutorial.models.Root')
       config.include('pyramid_chameleon')
       config.add_route('wiki_view', '/')
       config.add_route('wikipage_add', '/add')
       config.add_route('wikipage_view', '/{uid}')
       config.add_route('wikipage_edit', '/{uid}/edit')
       config.add_static_view('deform_static', 'deform:static/')
       config.scan('.views')
       return config.make_wsgi_app()
   

5. Make a command-line script at databases/tutorial/initialize_db.py to initialize the database:

   import os
   import sys
   import transaction
   
   from sqlalchemy import engine_from_config
   
   from pyramid.paster import (
       get_appsettings,
       setup_logging,
       )
   
   from .models import (
       DBSession,
       Page,
       Base,
       )
   
   
   def usage(argv):
       cmd = os.path.basename(argv[0])
       print('usage: %s <config_uri>\n'
             '(example: "%s development.ini")' % (cmd, cmd))
       sys.exit(1)
   
   
   def main(argv=sys.argv):
       if len(argv) != 2:
           usage(argv)
       config_uri = argv[1]
       setup_logging(config_uri)
       settings = get_appsettings(config_uri)
       engine = engine_from_config(settings, 'sqlalchemy.')
       DBSession.configure(bind=engine)
       Base.metadata.create_all(engine)
       with transaction.manager:
           model = Page(title='Root', body='<p>Root</p>')
           DBSession.add(model)
   

6. Now that we've got all the pieces ready, and because we changed setup.py, we now install all the goodies:

   $VENV/bin/pip install -e .
   

7. The script references some models in databases/tutorial/models.py:

   from pyramid.authorization import Allow, Everyone
   
   from sqlalchemy import (
       Column,
       Integer,
       Text,
       )
   
   from sqlalchemy.ext.declarative import declarative_base
   
   from sqlalchemy.orm import (
       scoped_session,
       sessionmaker,
       )
   
   from zope.sqlalchemy import register
   
   DBSession = scoped_session(sessionmaker())
   register(DBSession)
   Base = declarative_base()
   
   
   class Page(Base):
       __tablename__ = 'wikipages'
       uid = Column(Integer, primary_key=True)
       title = Column(Text, unique=True)
       body = Column(Text)
   
   
   class Root:
       __acl__ = [(Allow, Everyone, 'view'),
                  (Allow, 'group:editors', 'edit')]
   
       def __init__(self, request):
           pass
   

8. Let's run this console script, thus producing our database and table:

   $VENV/bin/initialize_tutorial_db development.ini
   
   2016-04-16 13:01:33,055 INFO  [sqlalchemy.engine.Engine][MainThread] SELECT CAST('test plain returns' AS VARCHAR(60)) AS anon_1
   2016-04-16 13:01:33,055 INFO  [sqlalchemy.engine.Engine][MainThread] ()
   2016-04-16 13:01:33,056 INFO  [sqlalchemy.engine.Engine][MainThread] SELECT CAST('test unicode returns' AS VARCHAR(60)) AS anon_1
   2016-04-16 13:01:33,056 INFO  [sqlalchemy.engine.Engine][MainThread] ()
   2016-04-16 13:01:33,057 INFO  [sqlalchemy.engine.Engine][MainThread] PRAGMA table_info("wikipages")
   2016-04-16 13:01:33,057 INFO  [sqlalchemy.engine.Engine][MainThread] ()
   2016-04-16 13:01:33,058 INFO  [sqlalchemy.engine.Engine][MainThread]
   CREATE TABLE wikipages (
          uid INTEGER NOT NULL,
          title TEXT,
          body TEXT,
          PRIMARY KEY (uid),
          UNIQUE (title)
   )
   
   
   2016-04-16 13:01:33,058 INFO  [sqlalchemy.engine.Engine][MainThread] ()
   2016-04-16 13:01:33,059 INFO  [sqlalchemy.engine.Engine][MainThread] COMMIT
   2016-04-16 13:01:33,062 INFO  [sqlalchemy.engine.Engine][MainThread] BEGIN (implicit)
   2016-04-16 13:01:33,062 INFO  [sqlalchemy.engine.Engine][MainThread] INSERT INTO wikipages (title, body) VALUES (?, ?)
   2016-04-16 13:01:33,063 INFO  [sqlalchemy.engine.Engine][MainThread] ('Root', '<p>Root</p>')
   2016-04-16 13:01:33,063 INFO  [sqlalchemy.engine.Engine][MainThread] COMMIT
   

9. With our data now driven by SQLAlchemy queries, we need to update our databases/tutorial/views.py:

   import colander
   import deform.widget
   
   from pyramid.httpexceptions import HTTPFound
   from pyramid.view import view_config
   
   from .models import DBSession, Page
   
   
   class WikiPage(colander.MappingSchema):
       title = colander.SchemaNode(colander.String())
       body = colander.SchemaNode(
           colander.String(),
           widget=deform.widget.RichTextWidget()
       )
   
   
   class WikiViews:
       def __init__(self, request):
           self.request = request
   
       @property
       def wiki_form(self):
           schema = WikiPage()
           return deform.Form(schema, buttons=('submit',))
   
       @property
       def reqts(self):
           return self.wiki_form.get_widget_resources()
   
       @view_config(route_name='wiki_view', renderer='wiki_view.pt')
       def wiki_view(self):
           pages = DBSession.query(Page).order_by(Page.title)
           return dict(title='Wiki View', pages=pages)
   
       @view_config(route_name='wikipage_add',
                    renderer='wikipage_addedit.pt')
       def wikipage_add(self):
           form = self.wiki_form.render()
   
           if 'submit' in self.request.params:
               controls = self.request.POST.items()
               try:
                   appstruct = self.wiki_form.validate(controls)
               except deform.ValidationFailure as e:
                   # Form is NOT valid
                   return dict(form=e.render())
   
               # Add a new page to the database
               new_title = appstruct['title']
               new_body = appstruct['body']
               DBSession.add(Page(title=new_title, body=new_body))
   
               # Get the new ID and redirect
               page = DBSession.query(Page).filter_by(title=new_title).one()
               new_uid = page.uid
   
               url = self.request.route_url('wikipage_view', uid=new_uid)
               return HTTPFound(url)
   
           return dict(form=form)
   
   
       @view_config(route_name='wikipage_view', renderer='wikipage_view.pt')
       def wikipage_view(self):
           uid = int(self.request.matchdict['uid'])
           page = DBSession.query(Page).filter_by(uid=uid).one()
           return dict(page=page)
   
   
       @view_config(route_name='wikipage_edit',
                    renderer='wikipage_addedit.pt')
       def wikipage_edit(self):
           uid = int(self.request.matchdict['uid'])
           page = DBSession.query(Page).filter_by(uid=uid).one()
   
           wiki_form = self.wiki_form
   
           if 'submit' in self.request.params:
               controls = self.request.POST.items()
               try:
                   appstruct = wiki_form.validate(controls)
               except deform.ValidationFailure as e:
                   return dict(page=page, form=e.render())
   
               # Change the content and redirect to the view
               page.title = appstruct['title']
               page.body = appstruct['body']
               url = self.request.route_url('wikipage_view', uid=uid)
               return HTTPFound(url)
   
           form = self.wiki_form.render(dict(
               uid=page.uid, title=page.title, body=page.body)
           )
   
           return dict(page=page, form=form)
   

10. Our tests in databases/tutorial/tests.py changed to include SQLAlchemy bootstrapping:

    import unittest
    import transaction
    
    from pyramid import testing
    
    
    def _initTestingDB():
        from sqlalchemy import create_engine
        from .models import (
            DBSession,
            Page,
            Base
            )
        engine = create_engine('sqlite://')
        Base.metadata.create_all(engine)
        DBSession.configure(bind=engine)
        with transaction.manager:
            model = Page(title='FrontPage', body='This is the front page')
            DBSession.add(model)
        return DBSession
    
    
    class WikiViewTests(unittest.TestCase):
        def setUp(self):
            self.session = _initTestingDB()
            self.config = testing.setUp()
    
        def tearDown(self):
            self.session.remove()
            testing.tearDown()
    
        def test_wiki_view(self):
            from tutorial.views import WikiViews
    
            request = testing.DummyRequest()
            inst = WikiViews(request)
            response = inst.wiki_view()
            self.assertEqual(response['title'], 'Wiki View')
    
    
    class WikiFunctionalTests(unittest.TestCase):
        def setUp(self):
            from pyramid.paster import get_app
            app = get_app('development.ini')
            from webtest import TestApp
            self.testapp = TestApp(app)
    
        def tearDown(self):
            from .models import DBSession
            DBSession.remove()
    
        def test_it(self):
            res = self.testapp.get('/', status=200)
            self.assertIn(b'Wiki: View', res.body)
            res = self.testapp.get('/add', status=200)
            self.assertIn(b'Add/Edit', res.body)
    

11. Run the tests in your package using pytest:

    $VENV/bin/pytest tutorial/tests.py -q
    ..
    2 passed in 1.41 seconds
    

12. Run your Pyramid application with:

    $VENV/bin/pserve development.ini --reload
    

13. Open http://localhost:6543/ in a browser.

Analysis

Let's start with the dependencies. We made the decision to use SQLAlchemy to talk to our database. We also, though, installed pyramid_tm and zope.sqlalchemy. Why?

Pyramid has a strong orientation towards support for transactions. Specifically, you can install a transaction manager into your application either as middleware or a Pyramid "tween". Then, just before you return the response, all transaction-aware parts of your application are executed.

This means Pyramid view code usually doesn't manage transactions. If your view code or a template generates an error, the transaction manager aborts the transaction. This is a very liberating way to write code.

The pyramid_tm package provides a "tween" that is configured in the development.ini configuration file. That installs it. We then need a package that makes SQLAlchemy, and thus the RDBMS transaction manager, integrate with the Pyramid transaction manager. That's what zope.sqlalchemy does.

Where do we point at the location on disk for the SQLite file? In the configuration file. This lets consumers of our package change the location in a safe (non-code) way. That is, in configuration. This configuration-oriented approach isn't required in Pyramid; you can still make such statements in your __init__.py or some companion module.

The initialize_tutorial_db is a nice example of framework support. You point your setup at the location of some [console_scripts], and these get generated into your virtual environment's bin directory. Our console script follows the pattern of being fed a configuration file with all the bootstrapping. It then opens SQLAlchemy and creates the root of the wiki, which also makes the SQLite file. Note the with transaction.manager part that puts the work in the scope of a transaction, as we aren't inside a web request where this is done automatically.

The models.py does a little bit of extra work to hook up SQLAlchemy into the Pyramid transaction manager. It then declares the model for a Page.

Our views have changes primarily around replacing our dummy dictionary-of-dictionaries data with proper database support: list the rows, add a row, edit a row, and delete a row.

Extra credit

1. Why all this code? Why can't I just type two lines and have magic ensue?

2. Give a try at a button that deletes a wiki page.