Sqlite / SQLAlchemy: how to enforce Foreign Keys?


The new version of SQLite has the ability to enforce Foreign Key constraints, but for the sake of backwards-compatibility, you have to turn it on for each database connection separately!

sqlite> PRAGMA foreign_keys = ON;

I am using SQLAlchemy — how can I make sure this always gets turned on?
What I have tried is this:

engine = sqlalchemy.create_engine('sqlite:///:memory:', echo=True)
engine.execute('pragma foreign_keys=on')

…but it is not working!…What am I missing?

I think my real problem is that I have more than one version of SQLite installed, and Python is not using the latest one!

>>> import sqlite3
>>> print sqlite3.sqlite_version

But I just downloaded 3.6.23 and put the exe in my project directory!
How can I figure out which .exe it’s using, and change it?

Asked By: Nick Perkins



I now have this working:

Download the latest sqlite and pysqlite2 builds as described above: make sure correct versions are being used at runtime by python.

import sqlite3   
import pysqlite2 
print sqlite3.sqlite_version   # should be
print pysqlite2.__path__       # eg C:\Python26\lib\site-packages\pysqlite2

Next add a PoolListener:

from sqlalchemy.interfaces import PoolListener
class ForeignKeysListener(PoolListener):
    def connect(self, dbapi_con, con_record):
        db_cursor = dbapi_con.execute('pragma foreign_keys=ON')

engine = create_engine(database_url, listeners=[ForeignKeysListener()])

Then be careful how you test if foreign keys are working: I had some confusion here. When using sqlalchemy ORM to add() things my import code was implicitly handling the relation hookups so could never fail. Adding nullable=False to some ForeignKey() statements helped me here.

The way I test sqlalchemy sqlite foreign key support is enabled is to do a manual insert from a declarative ORM class:

# example
ins = Coverage.__table__.insert().values(id = 99,
                                    description = 'Wrong',
                                    area = 42.0,
                                    wall_id = 99,  # invalid fkey id
                                    type_id = 99)  # invalid fkey_id

Here wall_id and type_id are both ForeignKey()‘s and sqlite throws an exception correctly now if trying to hookup invalid fkeys. So it works! If you remove the listener then sqlalchemy will happily add invalid entries.

I believe the main problem may be multiple sqlite3.dll’s (or .so) lying around.

Answered By: CarlS

I had the same problem before (scripts with foreign keys constraints were going through but actuall constraints were not enforced by the sqlite engine); got it solved by:

  1. downloading, building and installing the latest version of sqlite from here: sqlite-sqlite-amalgamation; before this I had sqlite 3.6.16 on my ubuntu machine; which didn’t support foreign keys yet; it should be 3.6.19 or higher to have them working.

  2. installing the latest version of pysqlite from here: pysqlite-2.6.0

after that I started getting exceptions whenever foreign key constraint failed

hope this helps, regards

Answered By: serge_gubenko

If you need to execute something for setup on every connection, use a PoolListener.

Answered By: Ants Aasma

For recent versions (SQLAlchemy ~0.7) the SQLAlchemy homepage says:

PoolListener is deprecated. Please refer to PoolEvents.

Then the example by CarlS becomes:

engine = create_engine(database_url)

def _fk_pragma_on_connect(dbapi_con, con_record):
    dbapi_con.execute('pragma foreign_keys=ON')

from sqlalchemy import event
event.listen(engine, 'connect', _fk_pragma_on_connect)
Answered By: conny

As a simpler approach if your session creation is centralised behind a Python helper function (rather than exposing the SQLA engine directly), you can just issue session.execute('pragma foreign_keys=on') before returning the freshly created session.

You only need the pool listener approach if arbitrary parts of your application may create SQLA sessions against the database.

Answered By: ncoghlan

From the SQLite dialect page:

SQLite supports FOREIGN KEY syntax when emitting CREATE statements for tables, however by default these constraints have no effect on the operation of the table.

Constraint checking on SQLite has three prerequisites:

  • At least version 3.6.19 of SQLite must be in use
  • The SQLite libary must be compiled without the SQLITE_OMIT_FOREIGN_KEY or SQLITE_OMIT_TRIGGER symbols enabled.
  • The PRAGMA foreign_keys = ON statement must be emitted on all connections before use.

SQLAlchemy allows for the PRAGMA statement to be emitted automatically for new connections through the usage of events:

from sqlalchemy.engine import Engine
from sqlalchemy import event

@event.listens_for(Engine, "connect")
def set_sqlite_pragma(dbapi_connection, connection_record):
    cursor = dbapi_connection.cursor()
    cursor.execute("PRAGMA foreign_keys=ON")
Answered By: shadowmatter

Building on the answers from conny and shadowmatter, here’s code that will check if you are using SQLite3 before emitting the PRAGMA statement:

from sqlalchemy import event
from sqlalchemy.engine import Engine
from sqlite3 import Connection as SQLite3Connection

@event.listens_for(Engine, "connect")
def _set_sqlite_pragma(dbapi_connection, connection_record):
    if isinstance(dbapi_connection, SQLite3Connection):
        cursor = dbapi_connection.cursor()
        cursor.execute("PRAGMA foreign_keys=ON;")
Answered By: Kiran Jonnalagadda

One-liner version of conny’s answer:

from sqlalchemy import event
event.listen(engine, 'connect', lambda c, _: c.execute('pragma foreign_keys=on'))
Answered By: xjcl

Enforce Foreign Key constraints for sqlite when using Flask + SQLAlchemy.

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

def create_app(config: str=None):
    app = Flask(__name__, instance_relative_config=True)
    if config is None:
        logger.debug('Using %s as configuration', config)


    # Ensure FOREIGN KEY for sqlite3
    if 'sqlite' in app.config['SQLALCHEMY_DATABASE_URI']:
        def _fk_pragma_on_connect(dbapi_con, con_record):  # noqa
            dbapi_con.execute('pragma foreign_keys=ON')

        with app.app_context():
            from sqlalchemy import event
            event.listen(db.engine, 'connect', _fk_pragma_on_connect)


Answered By: pgzmnk k