TypeError: Object of type RowProxy is not JSON serializable - Flask - session

Question

I am using SQLAlchemy to query the database from my Flask web-application using engine.After I do the SELECT Query and also do use fetchall object after ResultProxy is returned which ultimately returns RowProxy object and then I store in session.
Here is my code:
import os
from sqlalchemy import create_engine
from sqlalchemy.orm import scoped_session, sessionmaker
from flask import Flask, session
engine = create_engine(os.environ.get('DATABASE_URL'))
db = scoped_session(sessionmaker(bind=engine))
app = Flask(__name__)
app.secret_key = os.environ.get('SECRET_KEY')
#app.route('/')
def index():
session['list'] = db.execute("SELECT title,author,year FROM books WHERE year = 2011 LIMIT 4").fetchall()
print(session['list'])
return "<h1>hello world</h1>"
if __name__ == "__main__":
app.run(debug = True)
Here is the output:
[('Steve Jobs', 'Walter Isaacson', 2011), ('Legend', 'Marie Lu', 2011), ('Hit List', 'Laurell K. Hamilton', 2011), ('Born at Midnight', 'C.C. Hunter', 2011)]
Traceback (most recent call last):
File "C:\Users\avise\AppData\Local\Programs\Python\Python38\Lib\site-packages\flask\app.py", line 2463, in __call__
return self.wsgi_app(environ, start_response)
File "C:\Users\avise\AppData\Local\Programs\Python\Python38\Lib\site-packages\flask\app.py", line 2449, in wsgi_app
response = self.handle_exception(e)
File "C:\Users\avise\AppData\Local\Programs\Python\Python38\Lib\site-packages\flask\app.py", line 1866, in handle_exception
reraise(exc_type, exc_value, tb)
File "C:\Users\avise\AppData\Local\Programs\Python\Python38\Lib\json\encoder.py", line 179, in default
raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type RowProxy is not JSON serializable
The session item stores the data as i can see in output.But "hello world" is not rendered.
And if i replace the session variable by ordinary variable say x then it seems to be working.
But i think i need to use sessions so that my application will be used simultaneously by to users to display different things. So, how could i use sessions in this case or is there any other way?
Any help will be appreciated as I am new to Flask and web-development.

Answer 1

From what I understand about the Flask Session object is that it acts as a python dictionary; however values must be JSON serializable. In this case, just like the error suggests, the RowProxy object that is being returned by fetch all is not json serializable.
A solution to this problem would be to instead pass through a result of your query as a dictionary (which is JSON serializable).
It looks like the result of your query is returning a list of tuples so we can do the following:
res = db.execute("SELECT title,author,year FROM books WHERE year = 2011 LIMIT 4").fetchall()
user_books = {}
index = 0
for entry in res:
user_books[index] = {'title':res[index][0],
'author':res[index][1],
'year':res[index][2],
}
index += 1
session['list'] = user_books
A word of caution; however, is that since we are using the title of the book as a key, if there are two books with the same title, information may be overwritten, so consider using a unique id as the key.
Also note that the dictionary construction above would only work for the query you already have - if you added another column to the select statement you would have to edit the code to include the extra column information.