Schema Management

This document covers creating and dropping tables, managing indexes and constraints after the fact, and evolving a schema over time.

创建表

Create tables for a list of models with Database.create_tables()

db.create_tables([User, Tweet, Favorite])

By default Peewee uses CREATE TABLE IF NOT EXISTS, making it safe to call on application startup. To disable this, pass safe=False.

db.create_tables([User, Tweet, Favorite], safe=False)

To create a single table

Tweet.create_table()

Database.create_tables() respects foreign key dependencies: if Tweet references User, User’s table is created first regardless of the order in which they appear in the list.

Indexes declared in Meta.indexes and via Model.add_index() are created along with the table.

注意

A common pattern in web applications is to call db.create_tables(MODELS, safe=True) once at startup. This ensures all tables exist without failing on an already- initialized database. It does not apply schema changes - for that, see Schema Migrations.

Dropping Tables

db.drop_tables([User, Tweet, Favorite])

By default Peewee uses DROP TABLE IF EXISTS, making it safe to call multiple times. To disable this, pass safe=False.

db.drop_tables([User, Tweet, Favorite], safe=False)

Pass cascade=True (Postgresql and MySQL) to let the database handle dependency ordering

db.drop_tables([User, Tweet, Favorite], cascade=True)

To drop a single table

User.drop_table()

SchemaManager

SchemaManager provides finer-grained control over DDL operations. Each model exposes an instance at Model._schema.

Creating and dropping indexes independently

# Create just the indexes for a model (table already exists).
User._schema.create_indexes()

# Drop a specific index.
User._schema.drop_index(User.username)

Adding a foreign key constraint after table creation (useful when circular foreign keys are involved - see Circular foreign key dependencies in the models document)

# The table exists but the constraint was deferred.
User._schema.create_foreign_key(User.favorite_tweet)

注意

SQLite does not support adding foreign key constraints to existing tables. On SQLite, create_foreign_key will result in an OperationalError.

Truncating a table

User._schema.truncate_table()       # No cascade.
User._schema.truncate_table(cascade=True)   # Postgresql only.

另请参阅

SchemaManager API reference.

Schema Migrations

Peewee does not include a built-in migration system. For schema changes in an existing deployment - adding columns, dropping columns, renaming tables, modifying indexes - use one of the following approaches.

Playhouse migrate module

The migrate module in playhouse provides a set of helper functions for common schema changes, applied through a SchemaMigrator

from playhouse.migrate import *

db = SqliteDatabase(...)

migrator = SchemaMigrator.from_database(db)

first_name = TextField(default='')
last_name  = TextField(default='')

with db.atomic():
    migrate(
        migrator.add_column('person', 'first_name', first_name),
        migrator.add_column('person', 'last_name',  last_name),
        migrator.drop_column('person', 'name'),
    )

Supported operations

  • Add, rename, or drop columns.

  • Make columns nullable or not nullable.

  • Change a column’s type.

  • Rename a table.

  • Add or drop indexes and constraints.

  • Add or drop column default values.

另请参阅

Schema Migrations for in-depth examples and API reference.

Raw SQL migrations

For changes the migrate module does not cover, execute ALTER TABLE statements directly

with db.atomic():
    db.execute_sql('ALTER TABLE tweet ADD COLUMN view_count INTEGER DEFAULT 0')

SQLite limitations

SQLite has limited ALTER TABLE support. It supports ADD COLUMN and RENAME TABLE but not DROP COLUMN, RENAME COLUMN, or constraint changes in older versions (SQLite 3.35.0+ adds DROP COLUMN).

For more complex SQLite schema changes, the standard workaround is to

  1. Create a new table with the desired schema.

  2. Copy data with INSERT INTO new_table SELECT ... FROM old_table.

  3. Drop the old table.

  4. Rename the new table.

The playhouse migrate module transparently handles the above workaround for older SQLite versions.

Introspecting an Existing Schema

Database.get_tables() returns the names of all tables in the database

db.get_tables()
# ['user', 'tweet', 'favorite']

Database.get_columns() returns column metadata for a table as a list of ColumnMetadata instances

for col in db.get_columns('tweet'):
    print(col.name, col.data_type, col.null)

Database.get_indexes() returns index metadata as a list of IndexMetadata instances

for idx in db.get_indexes('user'):
    print(idx.name, idx.columns, idx.unique)

Database.get_foreign_keys() returns foreign key metadata as a list of ForeignKeyMetadata instances

for fk in db.get_foreign_keys('tweet'):
    print(fk.column, '->', fk.dest_table, fk.dest_column)

Database.get_views() returns a list of views in the database as a list of ViewMetadata instances

for view in db.get_views():
    print(view.name, view.sql)

Generating models from an existing database

The pwiz - Model Generator command-line tool introspects an existing database and emits Python model definitions

python -m pwiz -e postgresql my_database > models.py
python -m pwiz -e sqlite my_app.db > models.py

The generated models can be used directly or as a starting point for further customization.