2012-05-20T07:30:21Z

The Flask Mega-Tutorial, Part IV: Database

(Great news! There is a new version of this tutorial!)

This is the fourth article in the series in which I document my experience writing web applications in Python using the Flask microframework.

The goal of the tutorial series is to develop a decently featured microblogging application that demonstrating total lack of originality I have decided to call microblog.

NOTE: This article was revised in September 2014 to be in sync with current versions of Python and Flask.

Here is an index of all the articles in the series that have been published to date:

Recap

In the previous chapter of the series we created our login form, complete with submission and validation.

In this article we are going to create our database and set it up so that we can record our users in it.

To follow this chapter along you need to have the microblog app as we left it at the end of the previous chapter. Please make sure the app is installed and running.

Running Python scripts from the command line

In this chapter we are going to write a few scripts that simplify the management of our database. Before we get into that let's review how a Python script is executed on the command line.

If you are on Linux or OS X, then scripts have to be given executable permission, like this:

$ chmod a+x script.py

The script has a shebang line, which points to the interpreter that should be used. A script that has been given executable permission and has a shebang line can be executed simply like this:

./script.py <arguments>

On Windows, however, this does not work, and instead you have to provide the script as an argument to the chosen Python interpreter:

$ flask\Scripts\python script.py <arguments>

To avoid having to type the path to the Python interpreter you can add your microblog/flask/Scripts directory to the system path, making sure it appears before your regular Python interpreter. This can be temporarily achieved by activating the virtual environment with the following command:

$ flask\Scripts\activate

From now on, in this tutorial the Linux/OS X syntax will be used for brevity. If you are on Windows remember to convert the syntax appropriately.

Databases in Flask

We will use the Flask-SQLAlchemy extension to manage our application. This extension provides a wrapper for the SQLAlchemy project, which is an Object Relational Mapper or ORM.

ORMs allow database applications to work with objects instead of tables and SQL. The operations performed on the objects are translated into database commands transparently by the ORM. Knowing SQL can be very helpful when working with ORMs, but we will not be learning SQL in this tutorial, we will let Flask-SQLAlchemy speak SQL for us.

Migrations

Most database tutorials I've seen cover creation and use of a database, but do not adequately address the problem of updating a database as the application grows. Typically you end up having to delete the old database and create a new one each time you need to make updates, losing all the data. And if the data cannot be recreated easily you may be forced to write export and import scripts yourself.

Luckily, we have a much better option.

We are going to use SQLAlchemy-migrate to keep track of database updates for us. It adds a bit of work to get a database started, but that is a small price to pay for never having to worry about manual database migrations.

Enough theory, let's get started!

Configuration

For our little application we will use a sqlite database. The sqlite databases are the most convenient choice for small applications, as each database is stored in a single file and there is no need to start a database server.

We have a couple of new configuration items to add to our config file (file config.py):

import os
basedir = os.path.abspath(os.path.dirname(__file__))

SQLALCHEMY_DATABASE_URI = 'sqlite:///' + os.path.join(basedir, 'app.db')
SQLALCHEMY_MIGRATE_REPO = os.path.join(basedir, 'db_repository')

The SQLALCHEMY_DATABASE_URI is required by the Flask-SQLAlchemy extension. This is the path of our database file.

The SQLALCHEMY_MIGRATE_REPO is the folder where we will store the SQLAlchemy-migrate data files.

Finally, when we initialize our app we also need to initialize our database. Here is our updated package init file (file app/__init__.py):

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config.from_object('config')
db = SQLAlchemy(app)

from app import views, models

Note the two changes we have made to our init script. We are now creating a db object that will be our database, and we are also importing a new module called models. We will write this module next.

The database model

The data that we will store in our database will be represented by a collection of classes that are referred to as the database models. The ORM layer will do the translations required to map objects created from these classes into rows in the proper database table.

Let's start by creating a model that will represent our users. Using the WWW SQL Designer tool, I have made the following diagram to represent our users table:

users table

The id field is usually in all models, and is used as the primary key. Each user in the database will be assigned a unique id value, stored in this field. Luckily this is done automatically for us, we just need to provide the id field.

The nickname and email fields are defined as strings (or VARCHAR in database jargon), and their maximum lengths are specified so that the database can optimize space usage.

Now that we have decided what we want our users table to look like, the job of translating that into code is pretty easy (file app/models.py):

from app import db

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    nickname = db.Column(db.String(64), index=True, unique=True)
    email = db.Column(db.String(120), index=True, unique=True)

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

The User class that we just created contains several fields, defined as class variables. Fields are created as instances of the db.Column class, which takes the field type as an argument, plus other optional arguments that allow us, for example, to indicate which fields are unique and indexed.

The __repr__ method tells Python how to print objects of this class. We will use this for debugging.

Creating the database

With the configuration and model in place we are now ready to create our database file. The SQLAlchemy-migrate package comes with command line tools and APIs to create databases in a way that allows easy updates in the future, so that is what we will use. I find the command line tools a bit awkward to use, so instead I have written my own set of little Python scripts that invoke the migration APIs.

Here is a script that creates the database (file db_create.py):

#!flask/bin/python
from migrate.versioning import api
from config import SQLALCHEMY_DATABASE_URI
from config import SQLALCHEMY_MIGRATE_REPO
from app import db
import os.path
db.create_all()
if not os.path.exists(SQLALCHEMY_MIGRATE_REPO):
    api.create(SQLALCHEMY_MIGRATE_REPO, 'database repository')
    api.version_control(SQLALCHEMY_DATABASE_URI, SQLALCHEMY_MIGRATE_REPO)
else:
    api.version_control(SQLALCHEMY_DATABASE_URI, SQLALCHEMY_MIGRATE_REPO, api.version(SQLALCHEMY_MIGRATE_REPO))

Note how this script is completely generic. All the application specific pathnames are imported from the config file. When you start your own project you can just copy the script to the new app's directory and it will work right away.

To create the database you just need to execute this script (remember that if you are on Windows the command is slightly different):

./db_create.py

After you run the command you will have a new app.db file. This is an empty sqlite database, created from the start to support migrations. You will also have a db_repository directory with some files inside. This is the place where SQLAlchemy-migrate stores its data files. Note that we do not regenerate the repository if it already exists. This will allow us to recreate the database while leaving the existing repository if we need to.

Our first migration

Now that we have defined our model, we can incorporate it into our database. We will consider any changes to the structure of the app database a migration, so this is our first, which will take us from an empty database to a database that can store users.

To generate a migration I use another little Python helper script (file db_migrate.py):

#!flask/bin/python
import imp
from migrate.versioning import api
from app import db
from config import SQLALCHEMY_DATABASE_URI
from config import SQLALCHEMY_MIGRATE_REPO
v = api.db_version(SQLALCHEMY_DATABASE_URI, SQLALCHEMY_MIGRATE_REPO)
migration = SQLALCHEMY_MIGRATE_REPO + ('/versions/%03d_migration.py' % (v+1))
tmp_module = imp.new_module('old_model')
old_model = api.create_model(SQLALCHEMY_DATABASE_URI, SQLALCHEMY_MIGRATE_REPO)
exec(old_model, tmp_module.__dict__)
script = api.make_update_script_for_model(SQLALCHEMY_DATABASE_URI, SQLALCHEMY_MIGRATE_REPO, tmp_module.meta, db.metadata)
open(migration, "wt").write(script)
api.upgrade(SQLALCHEMY_DATABASE_URI, SQLALCHEMY_MIGRATE_REPO)
v = api.db_version(SQLALCHEMY_DATABASE_URI, SQLALCHEMY_MIGRATE_REPO)
print('New migration saved as ' + migration)
print('Current database version: ' + str(v))

The script looks complicated, but it doesn't really do much. The way SQLAlchemy-migrate creates a migration is by comparing the structure of the database (obtained in our case from file app.db) against the structure of our models (obtained from file app/models.py). The differences between the two are recorded as a migration script inside the migration repository. The migration script knows how to apply a migration or undo it, so it is always possible to upgrade or downgrade a database format.

While I have never had problems generating migrations automatically with the above script, I could see that sometimes it would be hard to determine what changes were made just by comparing the old and the new format. To make it easy for SQLAlchemy-migrate to determine the changes I never rename existing fields, I limit my changes to adding or removing models or fields, or changing types of existing fields. And I always review the generated migration script to make sure it is right.

It goes without saying that you should never attempt to migrate your database without having a backup, in case something goes wrong. Also never run a migration for the first time on a production database, always make sure the migration works correctly on a development database.

So let's go ahead and record our migration:

$ ./db_migrate.py

And the output from the script will be:

New migration saved as db_repository/versions/001_migration.py
Current database version: 1

The script shows where the migration script was stored, and also prints the current database version. The empty database version was version 0, after we migrated to include users we are at version 1.

Database upgrades and downgrades

By now you may be wondering why is it that important to go through the extra hassle of recording database migrations.

Imagine that you have your application in your development machine, and also have a copy deployed to a production server that is online and in use.

Let's say that for the next release of your app you have to introduce a change to your models, for example a new table needs to be added. Without migrations you would need to figure out how to change the format of your database, both in your development machine and then again in your server, and this could be a lot of work.

If you have database migration support, then when you are ready to release the new version of the app to your production server you just need to record a new migration, copy the migration scripts to your production server and run a simple script that applies the changes for you. The database upgrade can be done with this little Python script (file db_upgrade.py):

#!flask/bin/python
from migrate.versioning import api
from config import SQLALCHEMY_DATABASE_URI
from config import SQLALCHEMY_MIGRATE_REPO
api.upgrade(SQLALCHEMY_DATABASE_URI, SQLALCHEMY_MIGRATE_REPO)
v = api.db_version(SQLALCHEMY_DATABASE_URI, SQLALCHEMY_MIGRATE_REPO)
print('Current database version: ' + str(v))

When you run the above script, the database will be upgraded to the latest revision, by applying the migration scripts stored in the database repository.

It is not a common need to have to downgrade a database to an old format, but just in case, SQLAlchemy-migrate supports this as well (file db_downgrade.py):

#!flask/bin/python
from migrate.versioning import api
from config import SQLALCHEMY_DATABASE_URI
from config import SQLALCHEMY_MIGRATE_REPO
v = api.db_version(SQLALCHEMY_DATABASE_URI, SQLALCHEMY_MIGRATE_REPO)
api.downgrade(SQLALCHEMY_DATABASE_URI, SQLALCHEMY_MIGRATE_REPO, v - 1)
v = api.db_version(SQLALCHEMY_DATABASE_URI, SQLALCHEMY_MIGRATE_REPO)
print('Current database version: ' + str(v))

This script will downgrade the database one revision. You can run it multiple times to downgrade several revisions.

Database relationships

Relational databases are good at storing relations between data items. Consider the case of a user writing a blog post. The user will have a record in the users table, and the post will have a record in the posts table. The most efficient way to record who wrote a given post is to link the two related records.

Once a link between a user and a post is established there are two types of queries that we may need to use. The most trivial one is when you have a blog post and need to know what user wrote it. A more complex query is the reverse of this one. If you have a user, you may want to know all the posts that the user wrote. Flask-SQLAlchemy will help us with both types of queries.

Let's expand our database to store posts, so that we can see relationships in action. For this we go back to our database design tool and create a posts table:

users table

Our posts table will have the required id, the body of the post and a timestamp. Not much new there. But the user_id field deserves an explanation.

We said we wanted to link users to the posts that they write. The way to do that is by adding a field to the post that contains the id of the user that wrote it. This id is called a foreign key. Our database design tool shows foreign keys as a link between the foreign key and the id field of the table it refers to. This kind of link is called a one-to-many relationship, one user writes many posts.

Let's modify our models to reflect these changes (app/models.py):

from app import db

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    nickname = db.Column(db.String(64), index=True, unique=True)
    email = db.Column(db.String(120), index=True, unique=True)
    posts = db.relationship('Post', backref='author', lazy='dynamic')

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

class Post(db.Model):
    id = db.Column(db.Integer, primary_key = True)
    body = db.Column(db.String(140))
    timestamp = db.Column(db.DateTime)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'))

    def __repr__(self):
        return '<Post %r>' % (self.body)

We have added the Post class, which will represent blog posts written by users. The user_id field in the Post class was initialized as a foreign key, so that Flask-SQLAlchemy knows that this field will link to a user.

Note that we have also added a new field to the User class called posts, that is constructed as a db.relationship field. This is not an actual database field, so it isn't in our database diagram. For a one-to-many relationship a db.relationship field is normally defined on the "one" side. With this relationship we get a user.posts member that gets us the list of posts from the user. The first argument to db.relationship indicates the "many" class of this relationship. The backref argument defines a field that will be added to the objects of the "many" class that points back at the "one" object. In our case this means that we can use post.author to get the User instance that created a post. Don't worry if these details don't make much sense just yet, we'll see examples of this at the end of this article.

Let's record another migration with this change. Simply run:

$ ./db_migrate.py

And the script will respond:

New migration saved as db_repository/versions/002_migration.py
Current database version: 2

It isn't really necessary to record each little change to the database model as a separate migration, a migration is normally only recorded at significant points in the history of the project. We are doing more migrations than necessary here only to show how the migration system works.

Play time

We have spent a lot of time defining our database, but we haven't seen how it works yet. Since our app does not have database code yet let's make use of our brand new database in the Python interpreter.

So go ahead and fire up Python. On Linux or OS X:

flask/bin/python

Or on Windows:

flask\Scripts\python

Once in the Python prompt enter the following:

>>> from app import db, models
>>>

This brings our database and models into memory.

Let's create a new user:

>>> u = models.User(nickname='john', email='john@email.com')
>>> db.session.add(u)
>>> db.session.commit()
>>>

Changes to a database are done in the context of a session. Multiple changes can be accumulated in a session and once all the changes have been registered you can issue a single db.session.commit(), which writes the changes atomically. If at any time while working on a session there is an error, a call to db.session.rollback() will revert the database to its state before the session was started. If neither commit nor rollback are issued then the system by default will roll back the session. Sessions guarantee that the database will never be left in an inconsistent state.

Let's add another user:

>>> u = models.User(nickname='susan', email='susan@email.com')
>>> db.session.add(u)
>>> db.session.commit()
>>>

Now we can query what our users are:

>>> users = models.User.query.all()
>>> users
[<User u'john'>, <User u'susan'>]
>>> for u in users:
...     print(u.id,u.nickname)
...
1 john
2 susan
>>>

For this we have used the query member, which is available in all model classes. Note how the id member was automatically set for us.

Here is another way to do queries. If we know the id of a user we can find the data for that user as follows:

>>> u = models.User.query.get(1)
>>> u
<User u'john'>
>>>

Now let's add a blog post:

>>> import datetime
>>> u = models.User.query.get(1)
>>> p = models.Post(body='my first post!', timestamp=datetime.datetime.utcnow(), author=u)
>>> db.session.add(p)
>>> db.session.commit()

Here we set our timestamp in UTC time zone. All timestamps stored in our database will be in UTC. We can have users from all over the world writing posts and we need to use uniform time units. In a future tutorial we will see how to show these times to users in their local timezone.

You may have noticed that we have not set the user_id field of the Post class. Instead, we are storing a User object inside the author field. The author field is a virtual field that was added by Flask-SQLAlchemy to help with relationships, we have defined the name of this field in the backref argument to db.relationship in our model. With this information the ORM layer will know how to complete the user_id for us.

To complete this session, let's look at a few more database queries that we can do:

# get all posts from a user
>>> u = models.User.query.get(1)
>>> u
<User u'john'>
>>> posts = u.posts.all()
>>> posts
[<Post u'my first post!'>]

# obtain author of each post
>>> for p in posts:
...     print(p.id,p.author.nickname,p.body)
...
1 john my first post!

# a user that has no posts
>>> u = models.User.query.get(2)
>>> u
<User u'susan'>
>>> u.posts.all()
[]

# get all users in reverse alphabetical order
>>> models.User.query.order_by('nickname desc').all()
[<User u'susan'>, <User u'john'>]
>>>

The Flask-SQLAlchemy documentation is the best place to learn about the many options that are available to query the database.

Before we close, let's erase the test users and posts we have created, so that we can start from a clean database in the next chapter:

>>> users = models.User.query.all()
>>> for u in users:
...     db.session.delete(u)
...
>>> posts = models.Post.query.all()
>>> for p in posts:
...     db.session.delete(p)
...
>>> db.session.commit()
>>>

Final words

This was a long tutorial. We have learned the basics of working with a database, but we haven't incorporated the database into our application yet. In the next chapter we will put all we have learned about databases into practice when we look at our user login system.

In the meantime, if you haven't been writing the application along, you may want to download it in its current state:

Download microblog-0.4.zip.

Note that I have not included a database in the zip file above, but the repository with the migrations is there. To create a new database just use the db_create.py script, then use db_upgrade.py to upgrade the database to the latest revision.

I hope to see you next time!

Miguel

406 comments

  • #176 yan123 said 2013-12-19T19:30:46Z

    Running db_create.py; db_migrate.py creates db_repository/versions/001_migration.py without "user = Table(..." inside (using microblog-0.4.zip from current page with pre-deleted "db_repository" and app.db). Questions: 1) Should user object creation be added manually to 001_migrartion.py? 2) Page says "... SQLAlchemy-migrate creates a migration is by comparing the structure of the database (obtained in our case from file app.db) against the structure of our models (obtained from file app/models.py)..." How does script find out where model description is located? Should this be configured somewhere?

  • #177 Miguel Grinberg said 2013-12-21T18:31:02Z

    @yan123: for the migration to be generated correctly the changes to the database must only exist in the model definition. In other words, if you updated your database with these changes, then the models and the database are identical and the migration will be generated empty. To answer your questions: 1) No, sqlalchemy-migrate generates migration changes automatically 2) the models are loaded into the "db" database instance when you import the app package. As a side note, please make sure you are using sqlalchemy-migrate 0.7 with SQLAlchemy 0.7.9, I have not tested the recently released updates to sqlalchemy-migrate.

  • #178 yan123 said 2013-12-21T19:56:28Z

    @Miguel, thank You, finally figured out my misguidance, according to article, done this: 1) Fill in models.py with user object. 2) Run db_create.py Got app.db already containing "user" table (not empty as article says), so 001_migration.py has nothing new to add there.

    Possibly it was meant be as follows: 1) Create empty models.py first 2) Run db_create.py Obtain empty app.db, so, after that 3) Add "user" to models.py now, and 4) Run db_migrate.py would make script 'see' that there is something new in models.py and add this to 001_migration.py (thus making it look exactly like the one in microblog-0.4.zip)

  • #179 Ronald said 2014-01-02T13:18:01Z

    there's a new package called: alembic created by the developers of SQLAlchemy that can help you with the migrations.

    simple setup: From the console type: alembic init alembic This will great a default alembic.ini file.

    Within the file change the sqlalchemy.url. for me I changed it to: sqlalchemy.url = sqlite:///app.db

    Again from the console type: alembic revision -m "create user table" Your output will be something like Generating 22d3310cc98c_create_users_table.py ... done

    Once again in the console type: 'alembic upgrade head' you will get an output like: INFO [alembic.migration] Context impl SQLiteImpl. INFO [alembic.migration] Will assume non-transactional DDL. INFO [alembic.migration] Running upgrade None -> 22d3310cc98c, create users table

    And your done. You just created your app database.

  • #180 Ronald said 2014-01-02T13:31:44Z

    I forgot the tell something. your file (mine) 22d3310cc98c_create_users_table.py has a default empty value.

    def upgrade(): pass

    def downgrade(): pass

    you will have to tell alembic wat you want to upgrade. you can configure it so that alembic does it for you but that's a bit long to explain. see https://alembic.readthedocs.org/en/latest/tutorial.html#auto-generating-migrations for more information about this.

    for now replace def upgrade(): with

    def upgrade(): op.create_table( 'user', sa.Column('id', sa.Integer, primary_key=True), sa.Column('nickname', sa.VARCHAR(64), index=True, unique=True), sa.Column('email', sa.VARCHAR(120), index=True, unique=True), sa.Column('role', sa.Integer), )

  • #181 Tobi said 2014-01-05T20:30:50Z

    Thanks for the tutorial, waiting for the Flask book!

    I have a question regarding your newly released flask-migrate extension. How would I substitute your custom migration scripts by this extension? I'm not quite sure where to place the "manager.run" call (is it in run.py?) and how to take care of importing the models instead of explicitly defining them in the application itself. To me, it seems flask-migrate is pretty app-centric and has to reside in the same package which has the models in order to create the migrations. In this case, do I need to repeat the code for every package inside my application? Sure, there's a better way to do it. :-)

    Keep up the great work!

  • #182 Miguel Grinberg said 2014-01-06T03:47:40Z

    @Tobl: First of all, to use Flask-Migrate you also need to use Flask-Script. You will need to replace the code in run.py with the Flask-Script start up, which ends calling manager.run(). Flask-Migrate can be installed in this same script, and you shouldn't need to change anything else in the application. All Flask-Migrate needs is access to the "db" instance. To generate the first migration the easiest is to delete the database and then run the "db migrate" and "db upgrade" commands.

  • #183 Farshid P. said 2014-01-12T15:38:00Z

    Miguel first of all thank you for great tutorial, learned a lot. Now, I seem to have a error migrating the database.

    Before we continue let me say that I have SQLAlchemy==0.7.9 and sqlalchemy-migrate==0.7.2 installed.

    I have 17 versions in my database already. And so far everything seems to work fine. I recently changed models a lot and deleted few of them even changed around their variables which I suspect is causing the following error:

    Traceback (most recent call last):

    File "./db_migrate.py", line 11, in script = api.make_update_script_for_model(SQLALCHEMY_DATABASE_URI, SQLALCHEMY_MIGRATE_REPO, tmp_module.meta, db.metadata) File "", line 2, in make_update_script_for_model File "/Users/Farshid/Documents/Programming/GitHub/microblog/flask/lib/python2.7/site-packages/migrate/versioning/util/init.py", line 89, in catch_known_errors return f(a, kw) File "", line 2, in make_update_script_for_model File "/Users/Farshid/Documents/Programming/GitHub/microblog/flask/lib/python2.7/site-packages/migrate/versioning/util/init.py", line 159, in with_engine return f(*a, kw) File "/Users/Farshid/Documents/Programming/GitHub/microblog/flask/lib/python2.7/site-packages/migrate/versioning/api.py", line 321, in make_update_script_for_model engine, oldmodel, model, repository, *opts) File "/Users/Farshid/Documents/Programming/GitHub/microblog/flask/lib/python2.7/site-packages/migrate/versioning/script/py.py", line 69, in make_update_script_for_model genmodel.ModelGenerator(diff,engine).genB2AMigration() File "/Users/Farshid/Documents/Programming/GitHub/microblog/flask/lib/python2.7/site-packages/migrate/versioning/genmodel.py", line 197, in genB2AMigration for modelCol, databaseCol, modelDecl, databaseDecl in td.columns_different:

    ValueError: too many values to unpack

    When such problems arises in python, it usually means I am trying to enter x amount of items into a tuple with x+1 length. correct? So how do I go about fixing this problem for the database migration, and how do I prevent it from arising again in the future and doesn't mess up my app when it is online and live?

    Any help is greatly appreciated, also I welcome you to contact me via email if deemed necessarily. Thank you so much.

  • #184 Miguel Grinberg said 2014-01-12T19:09:27Z

    @Farshid: Migration frameworks cannot detect every change, some changes you may need to make corrections to the migration script. Large migrations with a lot of changes are usually not a good idea, it is a lot easier to detect smaller change sets. To solve this problem you may need to write this large migration script by hand, unfortunately.

  • #185 Farshid P. said 2014-01-13T11:35:19Z

    @Miguel: Thank you for the quick respond. Could you lead me in the right direction as to where I can learn how to write this migration my self ?

    Also if deleting the database and creating a new one would give me a temp fix, could you tell me how to delete the db completely and its migrations and create new one?

    I would really appreciate any help I get from you in this matter.

  • #186 Miguel Grinberg said 2014-01-14T05:14:54Z

    @Farshid: The documentation for sqlalchemy-migrate plus reading example auto-generated migrations should give you the elements that you need, I think. Another option you have is to start recording history again. If you eliminate the previous history you can generate an auto-migration as if it was the first one, assuming you don't care about keeping the previous history.

  • #187 Farshid P. said 2014-01-14T07:27:57Z

    Thank you Miguel.

    I deleted the history. At one point I even deleted the whole folder and recreated the database from scratch.

    Still getting the same error. Once I figure it out I will post it here for others to see, right now I haven't got a clue as to what it is that is causing this error.

  • #188 Miguel Grinberg said 2014-01-14T15:28:02Z

    @Farshid: since you seem to be okay with starting from scratch, you may want to give my Flask-Migrate extension, which is based on Alembic, a try.

  • #189 Matt Webster said 2014-01-17T03:10:10Z

    Hi Miguel, thanks for the amazing tutorial. I'm reading that SQLAlchemy is no longer being supported and that Alembic is a better choice. Thoughts?

  • #190 Miguel Grinberg said 2014-01-17T06:07:13Z

    @Matt: sqlalchemy-migrate did not receive updates for a long time, but not too long the ownership of the project was transferred to OpenStack, and they brought the project back to life. Despite this, I also use Alembic these days, I even wrote a Flask wrapper for it: https://github.com/miguelgrinberg/Flask-Migrate.

  • #191 Alex Morais said 2014-01-21T04:22:10Z

    Great tutorial, Miguel. I'm still on my way through it, now. I hate to bother but, even though I've followed the steps and looked for answers elsewhere, I am stuck on db_create.

    Has something changed or did I do something wrong during step 1? Firstly, I'm on Windows... When I run db_create.py, it fails internally on the import. It says I cannot import SchemaVisitor. I tried reexecuting the sqlalchemy steps of the installation process, but it didnt help, and it all seems to have been downloaded and installed successfully. Just querying to see if you know what I need to install to make this class available. Thank you.

  • #192 Alex Morais said 2014-01-21T04:27:58Z

    There was one that I didnt retry, and that seemed to have been overlooked in my original installation. Problem solved.

  • #193 askvictor said 2014-01-21T08:55:32Z

    I know it's two years later, but this is still a great tutorial for flask. However, migrations have moved on - it seems alembic is now the standard. I've used this tute along with http://michaelmartinez.in/basic-alembic-migrations-with-flask.html with success.

  • #194 xav said 2014-01-21T10:49:07Z

    Hi Miguel,

    Thanks for the very comprehensive tutorial. I am a bit stuck at the database creation part. When I run my db_create.py file, I get an import error:

    from migrate.versioning import api ... from migrate.changeset import ansisql ... from sqlalchemy.schema import SchemaVisitor ImportError: cannot import name SchemaVisitor

    I did not have any problems with any other tutorials I have done with SQLAlchemy. However, they usually used MySQL so it is the first time that I try to create the actual database from a script. Do you have any idea on why this would not work? Or if this is due to the numerous update all these plugins received?

    Thanks a lot

  • #195 Miguel Grinberg said 2014-01-22T15:10:06Z

    @askvictor: yes, since the time I wrote this article I have written Flask-Migrate, a wrapper extension for Alembic.

  • #196 Miguel Grinberg said 2014-01-22T15:12:37Z

    @xzav: first time I see this error. Make sure you have the correct versions of SQLAlchemy (0.7.9) and sqlalchemy-migrate (0.7.2).

  • #197 Kenny Landes said 2014-01-22T19:46:19Z

    I sent you props on Twitter last night, but wanted to thank you again for this tut. I've been learning Python and Flask for a project I'm about to start, and this tut is really great. I am able to understand what you're doing (most the time) and am excited about Flask.

  • #198 greg elofson said 2014-01-23T18:02:26Z

    Hi Miguel,

    I'm getting a complete fail when trying to create the database:

    greg-elofsons-imac:app greg$ python db_create.py Traceback (most recent call last): File "db_create.py", line 2, in from migrate.versioning import api ImportError: No module named migrate.versioning greg-elofsons-imac:app greg$ ./db_create.py bash: ./db_create.py: flask/bin/python: bad interpreter: No such file or directory greg-elofsons-imac:app greg$

    It's all apparently path related, and it seems that the config file is not automatically finding the path to the needed files when this line is invoked?:

    basedir = os.path.abspath(os.path.dirname(file))

  • #199 Miguel Grinberg said 2014-01-23T18:10:27Z

    @greg: looks like you have not created a virtual environment as explained in the first part of the series. You should have a "flask" folder with a private copy of the Python interpreter and a bunch of auxiliary packages installed and that appears to be missing.

  • #200 Matt G said 2014-02-01T18:22:04Z

    Miguel - thank you for this great tutorial!

    A question for you, in "from app import db" where is db specified? Is it the line "db = SQLAlchemy(app)" in init.py?