Assume table has three columns: username, password and no_of_logins.

When user tries to login, it’s checked for an entry with a query like

user = User.query.filter_by(username=form.username.data).first()

If password matches, he proceeds further. What I would like to do is count how many times the user logged in. Thus whenever he successfully logs in, I would like to increment the no_of_logins field and store it back to the user table. I’m not sure how to run update query with SqlAlchemy.

user.no_of_logins += 1
session.commit()

There are several ways to UPDATE using sqlalchemy

1) user.no_of_logins += 1
   session.commit()

2) session.query().\
       filter(User.username == form.username.data).\
       update({"no_of_logins": (User.no_of_logins +1)})
   session.commit()

3) conn = engine.connect()
   stmt = User.update().\
       values(no_of_logins=(User.no_of_logins + 1)).\
       where(User.username == form.username.data)
   conn.execute(stmt)

4) setattr(user, 'no_of_logins', user.no_of_logins+1)
   session.commit()

Examples to clarify the important issue in accepted answer’s comments

I didn’t understand it until I played around with it myself, so I figured there would be others who were confused as well. Say you are working on the user whose id == 6 and whose no_of_logins == 30 when you start.

# 1 (bad)
user.no_of_logins += 1
# result: UPDATE user SET no_of_logins = 31 WHERE user.id = 6

# 2 (bad)
user.no_of_logins = user.no_of_logins + 1
# result: UPDATE user SET no_of_logins = 31 WHERE user.id = 6

# 3 (bad)
setattr(user, 'no_of_logins', user.no_of_logins + 1)
# result: UPDATE user SET no_of_logins = 31 WHERE user.id = 6

# 4 (ok)
user.no_of_logins = User.no_of_logins + 1
# result: UPDATE user SET no_of_logins = no_of_logins + 1 WHERE user.id = 6

# 5 (ok)
setattr(user, 'no_of_logins', User.no_of_logins + 1)
# result: UPDATE user SET no_of_logins = no_of_logins + 1 WHERE user.id = 6

The point

By referencing the class instead of the instance, you can get SQLAlchemy to be smarter about incrementing, getting it to happen on the database side instead of the Python side. Doing it within the database is better since it’s less vulnerable to data corruption (e.g. two clients attempt to increment at the same time with a net result of only one increment instead of two). I assume it’s possible to do the incrementing in Python if you set locks or bump up the isolation level, but why bother if you don’t have to?

A caveat

If you are going to increment twice via code that produces SQL like SET no_of_logins = no_of_logins + 1, then you will need to commit or at least flush in between increments, or else you will only get one increment in total:

# 6 (bad)
user.no_of_logins = User.no_of_logins + 1
user.no_of_logins = User.no_of_logins + 1
session.commit()
# result: UPDATE user SET no_of_logins = no_of_logins + 1 WHERE user.id = 6

# 7 (ok)
user.no_of_logins = User.no_of_logins + 1
session.flush()
# result: UPDATE user SET no_of_logins = no_of_logins + 1 WHERE user.id = 6
user.no_of_logins = User.no_of_logins + 1
session.commit()
# result: UPDATE user SET no_of_logins = no_of_logins + 1 WHERE user.id = 6

With the help of user=User.query.filter_by(username=form.username.data).first() statement you will get the specified user in user variable.

Now you can change the value of the new object variable like user.no_of_logins += 1 and save the changes with the session‘s commit method.

I wrote telegram bot, and have some problem with update rows. Use this example, if you have Model

def update_state(chat_id, state):
    try:
        value = Users.query.filter(Users.chat_id == str(chat_id)).first()
        value.state = str(state)
        db.session.flush()
        db.session.commit()
        #db.session.close()
    except:
        print('Error in def update_state')

Why use db.session.flush()? That’s why >>> SQLAlchemy: What’s the difference between flush() and commit()?

How do I specify the column that I want in my query using a model (it selects all columns by default)? I know how to do this with the sqlalchmey session: session.query(self.col1), but how do I do it with with models? I can’t do SomeModel.query(). Is there a way?

You can use the with_entities() method to restrict which columns you’d like to return in the result. (documentation)

result = SomeModel.query.with_entities(SomeModel.col1, SomeModel.col2)

Depending on your requirements, you may also find deferreds useful. They allow you to return the full object but restrict the columns that come over the wire.

session.query().with_entities(SomeModel.col1)

is the same as

session.query(SomeModel.col1)

for alias, we can use .label()

session.query(SomeModel.col1.label('some alias name'))

You can use load_only function:

from sqlalchemy.orm import load_only

fields = ['name', 'addr', 'phone', 'url']
companies = session.query(SomeModel).options(load_only(*fields)).all()

You can use Model.query, because the Model (or usually its base class, especially in cases where declarative extension is used) is assigned Sesssion.query_property. In this case the Model.query is equivalent to Session.query(Model).

I am not aware of the way to modify the columns returned by the query (except by adding more using add_columns()).
So your best shot is to use the Session.query(Model.col1, Model.col2, ...) (as already shown by Salil).

You can use Query.values, Query.values

session.query(SomeModel).values('id', 'user')

An example here:

movies = Movie.query.filter(Movie.rating != 0).order_by(desc(Movie.rating)).all()

I query the db for movies with rating <> 0, and then I order them by rating with the higest rating first.

Take a look here: Select, Insert, Delete in Flask-SQLAlchemy

How can I update a row’s information?

For example I’d like to alter the name column of the row that has the id 5.

Retrieve an object using the tutorial shown in the Flask-SQLAlchemy documentation. Once you have the entity that you want to change, change the entity itself. Then, db.session.commit().

For example:

admin = User.query.filter_by(username='admin').first()
admin.email = 'my_new_email@example.com'
db.session.commit()

user = User.query.get(5)
user.name = 'New Name'
db.session.commit()

Flask-SQLAlchemy is based on SQLAlchemy, so be sure to check out the SQLAlchemy Docs as well.

There is a method update on BaseQuery object in SQLAlchemy, which is returned by filter_by.

admin = User.query.filter_by(username='admin').update(dict(email='my_new_email@example.com')))
db.session.commit()

The advantage of using update over changing the entity comes when there are many objects to be updated.

If you want to give add_user permission to all the admins,

rows_changed = User.query.filter_by(role='admin').update(dict(permission='add_user'))
db.session.commit()

Notice that filter_by takes keyword arguments (use only one =) as opposed to filter which takes an expression.

This does not work if you modify a pickled attribute of the model. Pickled attributes should be replaced in order to trigger updates:

from flask import Flask
from flask.ext.sqlalchemy import SQLAlchemy
from pprint import pprint

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqllite:////tmp/users.db'
db = SQLAlchemy(app)


class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(80), unique=True)
    data = db.Column(db.PickleType())

    def __init__(self, name, data):
        self.name = name
        self.data = data

    def __repr__(self):
        return '<User %r>' % self.username

db.create_all()

# Create a user.
bob = User('Bob', {})
db.session.add(bob)
db.session.commit()

# Retrieve the row by its name.
bob = User.query.filter_by(name='Bob').first()
pprint(bob.data)  # {}

# Modifying data is ignored.
bob.data['foo'] = 123
db.session.commit()
bob = User.query.filter_by(name='Bob').first()
pprint(bob.data)  # {}

# Replacing data is respected.
bob.data = {'bar': 321}
db.session.commit()
bob = User.query.filter_by(name='Bob').first()
pprint(bob.data)  # {'bar': 321}

# Modifying data is ignored.
bob.data['moo'] = 789
db.session.commit()
bob = User.query.filter_by(name='Bob').first()
pprint(bob.data)  # {'bar': 321}

Just assigning the value and committing them will work for all the data types but JSON and Pickled attributes. Since pickled type is explained above I’ll note down a slightly different but easy way to update JSONs.

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(80), unique=True)
    data = db.Column(db.JSON)

def __init__(self, name, data):
    self.name = name
    self.data = data

Let’s say the model is like above.

user = User("Jon Dove", {"country":"Sri Lanka"})
db.session.add(user)
db.session.flush()
db.session.commit()

This will add the user into the MySQL database with data {“country”:”Sri Lanka”}

Modifying data will be ignored. My code that didn’t work is as follows.

user = User.query().filter(User.name=='Jon Dove')
data = user.data
data["province"] = "south"
user.data = data
db.session.merge(user)
db.session.flush()
db.session.commit()

Instead of going through the painful work of copying the JSON to a new dict (not assigning it to a new variable as above), which should have worked I found a simple way to do that. There is a way to flag the system that JSONs have changed.

Following is the working code.

from sqlalchemy.orm.attributes import flag_modified
user = User.query().filter(User.name=='Jon Dove')
data = user.data
data["province"] = "south"
user.data = data
flag_modified(user, "data")
db.session.merge(user)
db.session.flush()
db.session.commit()

This worked like a charm. There is another method proposed along with this method here Hope I’ve helped some one.

How do I delete all rows in a single table using Flask-SQLAlchemy?

Looking for something like this:

>>> users = models.User.query.all()
>>> models.db.session.delete(users)

# but it errs out: UnmappedInstanceError: Class '__builtin__.list' is not mapped

Try delete:

models.User.query.delete()

From the docs: Returns the number of rows deleted, excluding any cascades.

DazWorrall’s answer is spot on. Here’s a variation that might be useful if your code is structured differently than the OP’s:

num_rows_deleted = db.session.query(Model).delete()

Also, don’t forget that the deletion won’t take effect until you commit, as in this snippet:

try:
    num_rows_deleted = db.session.query(Model).delete()
    db.session.commit()
except:
    db.session.rollback()

Flask-Sqlalchemy

Delete All Records

#for all records
db.session.query(Model).delete()
db.session.commit()

Deleted Single Row

here DB is the object Flask-SQLAlchemy class. It will delete all records from it and if you want to delete specific records then try filter clause in the query. ex.

#for specific value
db.session.query(Model).filter(Model.id==123).delete()
db.session.commit()

Delete Single Record by Object

record_obj = db.session.query(Model).filter(Model.id==123).first()
db.session.delete(record_obj)
db.session.commit()

https://flask-sqlalchemy.palletsprojects.com/en/2.x/queries/#deleting-records