Flask-SQLAlchemy check if row exists in table

Question

I have a Flask application which uses Flask-SQLAlchemy to connect to a MySQL database.

I would like to be able to check whether a row is present in a table. How woul

Answer 1

Wrap a .exists() query in another session.query() with a scalar() call at the end. SQLAlchemy will produce an optimized EXISTS query that returns True or False.

exists = db.session.query(
    db.session.query(User).filter_by(name='John Smith').exists()
).scalar()

SELECT EXISTS (SELECT 1 
FROM user 
WHERE user.name = ?) AS anon_1

While it's potentially more expensive due to the subquery, it's more clear about what's being queried. It may also be preferable over db.exists().where(...) because it selects a constant instead of the full row.