How can I select only one column using SQLAlchemy?

Note: If you call Model.query(params) you would see the following error

ERROR: 'BaseQuery' object is not callable

You need to call query(params) on session

example:

rows = session.query(Model.name).filter(Model.age >= 20).all()

Alternative:

You can do this using with_entities

example: To fetch names & age where age is greater than 20

rows = Model.query.with_entities(Model.name, Model.age).filter(Model.age >= 20).all()

To query the content of one column instead of the entire table flask-sqlalchemy, which I suppose can give you a hint about sqlalchemy itself would work gets you to query the session as you are doing, with a different syntax.

If your table looks something like:

class User(...):
    id = db.Column(db.Integer, primary_key=True)
    ...

You can query it with:

user_ids = session.query(User.id)
all_ids = user_ids.all()

This returns a list of all User Ids.

A Query object accepts entities to query as positional arguments, so just pass it User.id:

user_id = session.query(User.id).\
        filter(User.validation == request.cookies.get("validation")).\
        scalar()

scalar() returns the first element of the first result or None, if no rows were found. It raises MultipleResultsFound exception for multiple rows.

load_only() indicates that only the given column-based attributes of an entity should be loaded and all others, expect the identity, will be deferred. If you do need the whole User model object later, this can be the way to go. In that case your original query has to change to:

user = session.query(User).\
        filter(User.validation == request.cookies.get("validation")).\
        options(load_only("id")).\
        one()

one() returns exactly one result or raises an exception (0 or more than 1 result). If you accept None as a valid return value for "no user found", use one_or_none().

Note that predicates, the criteria of the WHERE clause, should not be passed to the Query object as entities, but added with filter().

To top it off, views in Flask expect that you return one of:

• a valid response object
• a string
• a (response, status, headers) tuple
• a WSGI application

The machinery will treat anything other than a response object, a string or a tuple as a WSGI application. In your original code you returned a Query object because of the missing call to scalar() or such and this was then treated as a WSGI app.