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

Forgive the hijacking but ... with the instructions given here I made the following WTForm validator to check uniqueness of the field

class Unique(object):
    def __init__(self, column, session, message="Already exists."):
        self.column = column
        self.session = session
        self.message = message

    def __call__(self, form, field):
        if field.data == field.object_data:
            return  # Field value equals to existing value. That's ok.
        model = self.column.class_
        query = model.query.filter(self.column == field.data).exists()
        if self.session.query(query).scalar():
            raise ValidationError(self.message)

It would be used like this

class Register(Form):
    email = EmailField('Email', [Unique(User.email, db.session)])

However, I would like to have API which does not need db session as a second param

class Register(Form):
    email = EmailField('Email', [Unique(User.email)])

Is there any way to get db session from model? Without session it seems to be impossible to avoid loading the entire object to check its existence.