Example app

../_images/tweepee.jpg

peewee ships with an example web app that runs on the Flask microframework. If you already have flask and its dependencies installed you should be good to go, otherwise install from the included requirements file.

cd example/
pip install -r requirements.txt

Running the example

After ensuring that flask, jinja2, werkzeug and sqlite3 are all installed, switch to the example directory and execute the run_example.py script:

python run_example.py

Diving into the code

Models

In the spirit of the ur-python framework, django, peewee uses declarative model definitions. If you’re not familiar with django, the idea is that you declare a class with some members which map directly to the database schema. For the twitter clone, there are just three models:

User:
represents a user account and stores the username and password, an email address for generating avatars using gravatar, and a datetime field indicating when that account was created
Relationship:
this is a “utility model” that contains two foreign-keys to the User model and represents “following”.
Message:
analagous to a tweet. this model stores the text content of the message, when it was created, and who posted it (foreign key to User).

If you like UML, this is basically what it looks like:

../_images/schema.jpg

Here is what the code looks like:

# create a peewee database instance -- our models will use this database to
# persist information
database = SqliteDatabase(DATABASE)

# model definitions -- the standard "pattern" is to define a base model class
# that specifies which database to use.  then, any subclasses will automatically
# use the correct storage. for more information, see:
# http://charlesleifer.com/docs/peewee/peewee/models.html#model-api-smells-like-django
class BaseModel(Model):
    class Meta:
        database = database

# the user model specifies its fields (or columns) declaratively, like django
class User(BaseModel):
    username = CharField()
    password = CharField()
    email = CharField()
    join_date = DateTimeField()

    class Meta:
        order_by = ('username',)

    # it often makes sense to put convenience methods on model instances, for
    # example, "give me all the users this user is following":
    def following(self):
        # query other users through the "relationship" table
        return User.select().join(
            Relationship, on=Relationship.to_user,
        ).where(Relationship.from_user == self)

    def followers(self):
        return User.select().join(
            Relationship, on=Relationship.from_user,
        ).where(Relationship.to_user == self)

    def is_following(self, user):
        return Relationship.select().where(
            (Relationship.from_user == self) &
            (Relationship.to_user == user)
        ).count() > 0

    def gravatar_url(self, size=80):
        return 'http://www.gravatar.com/avatar/%s?d=identicon&s=%d' % \
            (md5(self.email.strip().lower().encode('utf-8')).hexdigest(), size)


# this model contains two foreign keys to user -- it essentially allows us to
# model a "many-to-many" relationship between users.  by querying and joining
# on different columns we can expose who a user is "related to" and who is
# "related to" a given user
class Relationship(BaseModel):
    from_user = ForeignKeyField(User, related_name='relationships')
    to_user = ForeignKeyField(User, related_name='related_to')


# a dead simple one-to-many relationship: one user has 0..n messages, exposed by
# the foreign key.  because we didn't specify, a users messages will be accessible
# as a special attribute, User.message_set
class Message(BaseModel):
    user = ForeignKeyField(User)
    content = TextField()
    pub_date = DateTimeField()

    class Meta:
        order_by = ('-pub_date',)

peewee supports a handful of field types which map to different column types in sqlite. Conversion between python and the database is handled transparently, including the proper handling of None/NULL.

Note

You might have noticed that we created a BaseModel which sets the database, and then all the other models extend the BaseModel. This is a good way to make sure all your models are talking to the right database.

Creating the initial tables

In order to start using the models, its necessary to create the tables. This is a one-time operation and can be done quickly using the interactive interpreter.

Open a python shell in the directory alongside the example app and execute the following:

>>> from app import *
>>> create_tables()

The create_tables() method is defined in the app module and looks like this:

def create_tables():
    User.create_table()
    Relationship.create_table()
    Message.create_table()

Every model has a create_table() classmethod which runs a CREATE TABLE statement in the database. Usually this is something you’ll only do once, whenever a new model is added.

Note

Adding fields after the table has been created will required you to either drop the table and re-create it or manually add the columns using ALTER TABLE.

Note

If you want, you can use instead write User.create_table(True) and it will fail silently if the table already exists.

Connecting to the database

You may have noticed in the above model code that there is a class defined on the base model named Meta that sets the database attribute. peewee allows every model to specify which database it uses, defaulting to “peewee.db”. Since you probably want a bit more control, you can instantiate your own database and point your models at it. This is a peewee idiom:

# config
DATABASE = 'tweepee.db'

# ... more config here, omitted

database = SqliteDatabase(DATABASE) # tell our models to use "tweepee.db"

Because sqlite likes to have a separate connection per-thread, we will tell flask that during the request/response cycle we need to create a connection to the database. Flask provides some handy decorators to make this a snap:

@app.before_request
def before_request():
    g.db = database
    g.db.connect()

@app.after_request
def after_request(response):
    g.db.close()
    return response

Note

We’re storing the db on the magical variable g - that’s a flask-ism and can be ignored as an implementation detail. The meat of this code is in the idea that we connect to our db every request and close that connection every response. Django does the exact same thing.

Doing queries

In the User model there are a few instance methods that encapsulate some user-specific functionality, i.e.

  • following(): who is this user following?
  • followers(): who is following this user?

These methods are rather similar in their implementation but with one key difference:

def following(self):
    # query other users through the "relationship" table
    return User.select().join(
        Relationship, on=Relationship.to_user,
    ).where(Relationship.from_user == self)

def followers(self):
    return User.select().join(
        Relationship, on=Relationship.from_user,
    ).where(Relationship.to_user == self)

The queries end up looking like:

# following:
SELECT t1."id", t1."username", t1."password", t1."email", t1."join_date"
FROM "user" AS t1
INNER JOIN "relationship" AS t2
    ON t1."id" = t2."to_user_id"  # <-- joining on to_user_id
WHERE t2."from_user_id" = ?
ORDER BY t1."username" ASC

# followers
SELECT t1."id", t1."username", t1."password", t1."email", t1."join_date"
FROM user AS t1
INNER JOIN relationship AS t2
    ON t1."id" = t2."from_user_id"  # <-- joining on from_user_id
WHERE t2."to_user_id" = ?
ORDER BY t1."username" ASC

Creating new objects

So what happens when a new user wants to join the site? Looking at the business end of the join() view, we can that it does a quick check to see if the username is taken, and if not executes a create().

Much like the create() method, all models come with a built-in method called get_or_create() which is used when one user follows another:

Relationship.get_or_create(
    from_user=session['user'], # <-- the logged-in user
    to_user=user, # <-- the user they want to follow
)

Doing subqueries

If you are logged-in and visit the twitter homepage, you will see tweets from the users that you follow. In order to implement this, it is necessary to do a subquery:

# python code
messages = Message.select().where(
    Message.user << user.following()
)

Results in the following SQL query:

SELECT t1."id", t1."user_id", t1."content", t1."pub_date"
FROM "message" AS t1
WHERE t1."user_id" IN (
    SELECT t2."id"
    FROM "user" AS t2
    INNER JOIN "relationship" AS t3
        ON t2."id" = t3."to_user_id"
    WHERE t3."from_user_id" = ?
    ORDER BY t1."username" ASC
)

peewee supports doing subqueries on any ForeignKeyField or PrimaryKeyField.

What else is of interest here?

There are a couple other neat things going on in the example app that are worth mentioning briefly.

  • Support for paginating lists of results is implemented in a simple function called object_list (after it’s corollary in Django). This function is used by all the views that return lists of objects.

    def object_list(template_name, qr, var_name='object_list', **kwargs):
        kwargs.update(
            page=int(request.args.get('page', 1)),
            pages=qr.count() / 20 + 1
        )
        kwargs[var_name] = qr.paginate(kwargs['page'])
        return render_template(template_name, **kwargs)
    
  • Simple authentication system with a login_required decorator. The first function simply adds user data into the current session when a user successfully logs in. The decorator login_required can be used to wrap view functions, checking for whether the session is authenticated and if not redirecting to the login page.

    def auth_user(user):
        session['logged_in'] = True
        session['user'] = user
        session['username'] = user.username
        flash('You are logged in as %s' % (user.username))
    
    def login_required(f):
        @wraps(f)
        def inner(*args, **kwargs):
            if not session.get('logged_in'):
                return redirect(url_for('login'))
            return f(*args, **kwargs)
        return inner
    
  • Return a 404 response instead of throwing exceptions when an object is not found in the database.

    def get_object_or_404(model, **kwargs):
        try:
            return model.get(**kwargs)
        except model.DoesNotExist:
            abort(404)
    

Note

Like these snippets and interested in more? Check out flask-peewee - a flask plugin that provides a django-like Admin interface, RESTful API, Authentication and more for your peewee models.