Model definitions

We’ll use the following model definitions for our examples:

import datetime
from peewee import *
 
 
db = SqliteDatabase(':memory:')
 
class BaseModel(Model):
    class Meta:
        database = db
 
class User(BaseModel):
    username = TextField()
 
class Tweet(BaseModel):
    content = TextField()
    timestamp = DateTimeField(default=datetime.datetime.now)
    user = ForeignKeyField(User, backref='tweets')
 
class Favorite(BaseModel):
    user = ForeignKeyField(User, backref='favorites')
    tweet = ForeignKeyField(Tweet, backref='favorites')

Peewee uses ForeignKeyField to define foreign-key relationshipsbetween models. Every foreign-key field has an implied back-reference, which isexposed as a pre-filtered Select query using the providedbackref attribute.

Creating test data

To follow along with the examples, let’s populate this database with some testdata:

def populate_test_data():
    db.create_tables([User, Tweet, Favorite])
 
    data = (
        ('huey', ('meow', 'hiss', 'purr')),
        ('mickey', ('woof', 'whine')),
        ('zaizee', ()))
    for username, tweets in data:
        user = User.create(username=username)
        for tweet in tweets:
            Tweet.create(user=user, content=tweet)
 
    # Populate a few favorites for our users, such that:
    favorite_data = (
        ('huey', ['whine']),
        ('mickey', ['purr']),
        ('zaizee', ['meow', 'purr']))
    for username, favorites in favorite_data:
        user = User.get(User.username == username)
        for content in favorites:
            tweet = Tweet.get(Tweet.content == content)
            Favorite.create(user=user, tweet=tweet)

This gives us the following:

Attention

In the following examples we will be executing a number of queries. If youare unsure how many queries are being executed, you can add the followingcode, which will log all queries to the console:

import logging
logger = logging.getLogger('peewee')
logger.addHandler(logging.StreamHandler())
logger.setLevel(logging.DEBUG)

Note

In SQLite, foreign keys are not enabled by default. Most things, includingthe Peewee foreign-key API, will work fine, but ON DELETE behaviour will beignored, even if you explicitly specify on_delete in yourForeignKeyField. In conjunction with the defaultAutoField behaviour (where deleted record IDs can be reused),this can lead to subtle bugs. To avoid problems, I recommend that youenable foreign-key constraints when using SQLite, by settingpragmas={'foreign_keys': 1} when you instantiate SqliteDatabase.

# Ensure foreign-key constraints are enforced.
db = SqliteDatabase('my_app.db', pragmas={'foreign_keys': 1})