Example app¶

We’ll be building a simple twitter-like site. The source code for the example can be found in the examples/twitter directory. You can also browse the source-code on github.

Tip

There is also an example blog app, however it is not covered in this guide.

The example app uses the flask web framework. You will need to install it to run the example:

pip install flask

Running the Example¶

After ensuring that flask is installed, cd into the twitter example directory and execute the run_example.py script:

python run_example.py

The example app will be accessible at http://localhost:5000/

Code Structure¶

For simplicity all example code is contained within a single module, examples/twitter/app.py. For a guide on structuring larger Flask apps with peewee, check out Structuring Flask Apps.

Models¶

peewee uses declarative model definitions. Declare a model class for each table. The model class then defines one or more field attributes which correspond to the table’s columns. For the twitter clone, there are 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 stores which users follow one another.
Message:: Analogous to a tweet. The Message model stores the text content of the tweet, when it was created, and who posted it (foreign key to User).

If you like UML, these are the tables and relationships:

In order to create these models we need:

declare a SqliteDatabase object
declare our model classes
declare columns as Field instances on the model classes

# 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.
class BaseModel(Model):
    class Meta:
        database = database

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

# 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, backref='relationships')
    to_user = ForeignKeyField(User, backref='related_to')

    class Meta:
        # `indexes` is a tuple of 2-tuples, where the 2-tuples are
        # a tuple of column names to index and a boolean indicating
        # whether the index is unique or not.
        indexes = (
            # Specify a unique multi-column index on from/to-user.
            (('from_user', 'to_user'), True),
        )

# a dead simple one-to-many relationship: one user has 0..n messages, exposed by
# the foreign key. a users messages will be accessible as a special attribute,
# User.messages.
class Message(BaseModel):
    user = ForeignKeyField(User, backref='messages')
    content = TextField()
    pub_date = DateTimeField()

Note

Note that we create a BaseModel class that simply defines what database we would like to use. All other models then extend this class and will also use the correct database connection.

Peewee supports many different field types which map to different column types commonly supported by database engines. Conversion between python types and those used in the database is handled transparently, allowing you to use the following in your application:

Strings (unicode or otherwise)
Integers, floats, and Decimal numbers.
Boolean values
Dates, times and datetimes
None (NULL)
Binary data

Creating 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. We can create a small helper function to accomplish this:

def create_tables():
    with database:
        database.create_tables([User, Relationship, Message])

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

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

Attention

If you encounter an ImportError it means that either flask or peewee was not found and may not be installed correctly. Check the Installing and Testing document for instructions on installing peewee.

Every model has a create_table() classmethod which runs a SQL CREATE TABLE statement in the database. This method will create the table, including:

columns
foreign-key constraints
indexes
sequences
check constraints

Usually this is something you’ll only do once, when a new model is added.

Peewee provides a helper method Database.create_tables() which will resolve inter-model dependencies and call create_table() on each model, ensuring the tables are created in order.

Note

Adding, removing or modifying fields after the table has been created will require you to either:

drop the table and re-create it, OR
manually add, drop or modify the columns, OR
use the migration tools to script your changes.

Database Connection¶

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. There are many Meta options you can specify which control the behavior of your model.

This is a peewee idiom:

DATABASE = 'tweepee.db'

# Create a database instance that will manage the connection and
# execute queries
database = SqliteDatabase(DATABASE)

# Create a base-class all our models will inherit, which defines
# the database we'll be using.
class BaseModel(Model):
    class Meta:
        database = database

When developing a web application, it’s common to:

Open a connection when a request starts.
Run your request-handler.
Close the connection before returning the response.

You should always manage your connections explicitly. For instance, if you are using a connection pool, connections will only be recycled correctly if you call connect() and close().

Flask provides connection setup/teardown hooks via decorators:

@app.before_request
def before_request():
    database.connect()

@app.teardown_request
def teardown_request(exc):
    if not database.is_closed():
        database.close()

Making Queries¶

In the User model there are a few instance methods that encapsulate some user-specific functionality:

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

These methods are similar in their implementation but with an important difference in the SQL JOIN and WHERE clauses:

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

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

Storing Data¶

When a new user wants to join the site we need to make sure the username is available, and if so, create a new User record. Looking at the join() view, we can see that our application attempts to create the User using Model.create(). User.username field has a unique constraint, so if the username is taken the database will raise an IntegrityError.

try:
    with database.atomic():
        # Attempt to create the user. If the username is taken, due to the
        # unique constraint, the database will raise an IntegrityError.
        user = User.create(
            username=request.form['username'],
            password=md5(request.form['password']).hexdigest(),
            email=request.form['email'],
            join_date=datetime.datetime.now())

    # mark the user as being 'authenticated' by setting the session vars
    auth_user(user)
    return redirect(url_for('homepage'))

except IntegrityError:
    flash('That username is already taken')

We will use a similar approach when a user wishes to follow someone. To indicate a following relationship, we create a row in the Relationship table pointing from one user to another. Due to the unique index on (from_user, to_user), we will be sure not to end up with duplicate rows:

user = get_object_or_404(User, username=username)
try:
    with database.atomic():
        Relationship.create(
            from_user=get_current_user(),
            to_user=user)
except IntegrityError:
    pass

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 cleanly, we can use a subquery:

Note

user.following() will automatically only select User.id when it used in a subquery.

# python code
user = get_current_user()
messages = (Message
            .select()
            .where(Message.user.in_(user.following()))
            .order_by(Message.pub_date.desc()))

This code corresponds to 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" = ?
)

More Examples¶

There are more examples included in the peewee examples directory, including:

Example blog app using Flask and peewee. Also see accompanying blog post.
An encrypted command-line diary. There is a companion blog post you might enjoy as well.
Analytics web-service (like a lite version of Google Analytics). Also check out the companion blog post.

Example app¶

Running the Example¶

Code Structure¶

Models¶

Creating Tables¶

Database Connection¶

Making Queries¶

Storing Data¶

Subqueries¶

Other Topics¶

More Examples¶

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