Flask-SQLAlchemy check if row exists in table

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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 would I modify a query like so to check the row exists:

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

I found a solution on this question which uses SQLAlchemy but does not seem to fit with the way Flask-SQLAlchemy works:

from sqlalchemy.sql import exists    
print session.query(exists().where(User.email == '...')).scalar()

Thanks.

Since you only want to see if the user exists, you don't want to query the entire object. Just query the id, it exists if the scalar return is not None.

exists = db.session.query(User.id).filter_by(name='davidism').scalar() is not None
SELECT user.id AS user_id 
FROM user 
WHERE user.name = ?

The second query you showed also works fine, Flask-SQLAlchemy does nothing to prevent any type of query that SQLAlchemy can make. This returns False or True instead of None or an id like above, but it is slightly more expensive because it uses a subquery.

exists = db.session.query(db.exists().where(User.name == 'davidism')).scalar()
SELECT EXISTS (SELECT * 
FROM user 
WHERE user.name = ?) AS anon_1

python - Flask-SQLAlchemy check if row exists in table, 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. I would like to be able to check whether a row is present in a table. How would I modify a query like so to check the row exists: db.session.query(User).filter_by(name='John Smith') I found a solution on this question which uses SQLAlchemy but does not seem to fit with the way Flask-SQLAlchemy works:

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.

Flask-SQLAlchemy check if row exists in table, 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. 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 would I modify a query like so to check the row exists: db.session.query(User).filter_by(name='John Smith') I found a solution on this question which uses SQLAlchemy […]

bool(User.query.filter_by(name='John Smith').first())

It will return False if objects with this name doesn't exist and True if it exists.

Python Examples of sqlalchemy.exists, This page shows Python examples of sqlalchemy.exists. You may want to check out the right sidebar which shows the related API usage. Table": """ Returns table or creates one if the table name is not in the table list""" import boto3� 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 would I modify a query like so to check the row exists: db.session.query(User).filter_by(name='John Smith') I found a solution on this question this question

Think there is a typo in davidism's answer, this works for me:

exists = db.session.query(**User**).filter_by(name='davidism').scalar() is not None

Flask-SQLAlchemy Check if row exists, So I am trying to check if a row exists in a table with Flask-SQLAlchemy, and so far nothing has helped.My current code:@app.route(' Flask-SQLAlchemy check if table exists in database. I see similar problems, but I try not to succeed. Flask-SQLAlchemy check if row exists in table. I have create a table object ,like this: <class'flask_sqlalchemy.XXX'>, now how to check the object if exists in database. I do many try: eg:

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.

SQLAlchemy, One way we could find out if our Keyword table has any of a certain set of Above we ask "do any rows exist, where the ID of the recipe table matches the recipe I hope examples like Reddit continue to illustrate that Python presents the best� Flask-SQLAlchemy check if row exists in table. Hot Network Questions usage of "entertainment" Append a local file to a remote file over ssh Set a variable to the result of a division and subtraction command

sqlite: add an entry if not exists : flask, You basically just query first, and if it does not exist, then create it. Sharp - automatic API generation library for Python Flask and JavaScript The problem is that when there is a lot of rows(1M) in impressions table count function takes 0.6s� SQL Check if row exists in table Check if row exists in table Before you insert, update or delete rows from a sql table, you may need to know if there are any records in the table. Check if there are rows in the table using TOP, COUNT, EXISTS or NOT EXISTS.

关于python:Flask-SQLAlchemy检查表中是否存在行| 码农家园, SQLAlchemy's ORM query API simplifies the way we write database queries. The last part of our query determines how many rows to return, and the nature of if one exists, None if no values exist, or raises an exception if multiple records are Join records from different tables and deserialize records. Flask-SQLAlchemy check if row exists in table. 4. Reflecting tables with Flask-SQLAlchemy raises RuntimeError: application not registered. 13. Flask-SQLAlchemy - how do sessions work with multiple databases? 3. It is necessary use db.init_app or SQLAlchem

Constructing Database Queries with SQLAlchemy, Select, Insert, Delete�. Now that you have declared models it's time to query the data from the database. We will be using the model definitions from the� Thanks. python flask sqlalchemy flask-sqlalchemy | this question edited May 23 at 12:26 Community ♦ 1 1 asked Oct 4 '15 at 21:14 Pav Sidhu 1,081 1 16 45 In what way does the first query not check that the row exists? – Daniel Roseman Oct 4 '15 at 21:17 I would like the query to return True or False if the row exists. – Pav Sidhu Oct 4 '15

Comments
  • In what way does the first query not check that the row exists?
  • I would like the query to return True or False if the row exists.
  • simply use count, query.count()
  • How can you do this for ms sql server? I am using the standard sql alchemy @lyschoening
  • I found this answer useful, but I had to change it to bool(session.query(User).filter_by(name='John Smith').first())
  • This should be a separate question, not an answer