user2986242 user2986242 - 1 month ago 26
Python Question

sqlalchemy.orm.exc.UnmappedInstanceError in flask

I have been reading the SQLAlchemy docs, but I don't understand them. The error (UnmappedInstanceError) says something isn't mapped. What isn't mapped? I really don't get sqlalchemy and I want to go back to using naked sqlite, but so many people recommend this, so I thought I should learn it. Here is the traceback:

File "C:\Users\Me\repos\mandj\venv\lib\site-packages\flask\app.py", line 1836, in __call__
return self.wsgi_app(environ, start_response)

File "C:\Users\Me\repos\mandj\venv\lib\site-packages\flask\app.py", line 1820, in wsgi_app
response = self.make_response(self.handle_exception(e))

File "C:\Users\Me\repos\mandj\venv\lib\site-packages\flask\app.py", line 1403, in handle_exception
reraise(exc_type, exc_value, tb)

File "C:\Users\Me\repos\mandj\venv\lib\site-packages\flask\app.py", line 1817, in wsgi_app
response = self.full_dispatch_request()

File "C:\Users\Me\repos\mandj\venv\lib\site-packages\flask\app.py", line 1477, in full_dispatch_request
rv = self.handle_user_exception(e)

File "C:\Users\Me\repos\mandj\venv\lib\site-packages\flask\app.py", line 1381, in handle_user_exception
reraise(exc_type, exc_value, tb)

File "C:\Users\Me\repos\mandj\venv\lib\site-packages\flask\app.py", line 1475, in full_dispatch_request
rv = self.dispatch_request()

File "C:\Users\Me\repos\mandj\venv\lib\site-packages\flask\app.py", line 1461, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)

File "C:\Users\Me\repos\mandj2\app\views.py", line 170, in add_manentry
db.session.add(q)

File "C:\Users\Me\repos\mandj\venv\lib\site-packages\sqlalchemy\orm\scoping.py", line 149, in do
return getattr(self.registry(), name)(*args, **kwargs)

File "C:\Users\Me\repos\mandj\venv\lib\site-packages\sqlalchemy\orm\session.py", line 1452, in add
raise exc.UnmappedInstanceError(instance)

UnmappedInstanceError: Class '__builtin__.unicode' is not mapped


Here is the applicable code:

@app.route('/addm', methods=['POST'])
def add_mentry():
if not session.get('logged_in'):
abort(401)
form = MForm(request.form)
filename = ""
if request.method == 'POST':
cover = request.files['cover']
if cover and allowed_file(cover.filename):
filename = secure_filename(cover.filename)
cover = cover.save(os.path.join(app.config['UPLOAD_FOLDER'], filename))

q = request.form['name']
# do for 12 more fields
db.session.add(q)
db.session.commit()
flash('New entry was successfully posted')
return redirect(url_for('moutput'))

Answer
q = request.form['name']
# do for 12 more fields
db.session.add(q)

request.form['name'] will return a unicode value. Then, you do...

db.session.add(q)

The goal of the session is to keep track of Entities (Python objects), not individual unicode values as you seem to be trying to do it (See here for more on what the session does). Thus, you should be adding objects that have a mapping (such as a User object as shown in the "Mapping" section of the ORM tutorial), but you're actually passing a simple unicode value

What you're using is just one part of SQLAlchemy: the ORM (Object-Relational Mapper). The ORM will try to do things like allow you to create a new python object and have the SQL automatically generaeted by "adding" the object to the session..

a = MyEntity()
session.add(a)
session.commit() # Generates SQL to do an insert for the table that MyEntity is for

Keep in mind that you can use SQLAlchemy without using the ORM functionality. You could just do db.execute('INSERT...', val1, val2) to replace your already "naked" SQL. SQLAlchemy will provide you connection pooling, etc (although if you're using SQLite, you probably don't care about connection pooling).

If you want to understand Flask-SQLAlchemy, I would first recommend understanding how SQLAlchemy works (especially the ORM side) by trying it out using simple scripts (as shown in the tutorials. Then you'll understand how Flask-SQLAlchemy would work with it.