sqlalchemy exists for query

How do I check whether data in a query exists?

For example:

users_query = User.query.filter_by(email='x@x.com')

How I can check whether users with that email exist?

I can check this with

use开发者_开发百科rs_query.count()

but how to check it with exists?

The following solution is a bit simpler:

from sqlalchemy.sql import exists

print session.query(exists().where(User.email == '...')).scalar()

The most acceptable and readable option for me is

session.query(<Exists instance>).scalar()

like

session.query(User.query.filter(User.id == 1).exists()).scalar()

which returns True or False.

There is no way that I know of to do this using the orm query api. But you can drop to a level lower and use exists from sqlalchemy.sql.expression:

from sqlalchemy.sql.expression import select, exists

users_exists_select = select((exists(users_query.statement),)) 
print engine.execute(users_exists_select).scalar()

2021 Answer for SqlAlchemy 1.4

Refrain from calling .query and instead use select directly chained with .exists as follows:

from sqlalchemy import select

stmt = select(User).where(User.email=="x@x.com").exists()

Source: https://docs.sqlalchemy.org/en/14/core/selectable.html?highlight=exists#sqlalchemy.sql.expression.exists

For SQL Server, I had to do this:

from sqlalchemy.sql.expression import literal

result = session.query(literal(True)).filter(
    session.query(User)
    .filter_by(email='...')
    .exists()
).scalar()
print(result is not None)

# Resulting query:
# SELECT 1
# WHERE EXISTS (SELECT 1 
# FROM User
# WHERE User.email = '...')

But it's much simpler without EXISTS:

result = (
    session.query(literal(True))
    .filter(User.email == '...')
    .first()
)
print(result is not None)

# Resulting query:
# SELECT TOP 1 1
# FROM User
# WHERE User.email = '...'

The easiest way in SQLAlchemy 1.4/2.0 with the new unified API:

from sqlalchemy import exists


session.scalar(
    exists()
    .where(User.email == 'x@x.com')
    .select()
)

it can be done:

from sqlalchemy import select

user = session.scalars(
    select(User).where(User.email=="x@x.com")
).first()

if user:
    pass
else:
    pass