Alembic - Migrations for SQLalchemy (and Flask)
What I recently came across, was the problem that I could not run database upgrades without a lot of pain. Everytime I wanted to change something on the production server or move stuff from the development to another server, I had either to reset the database (only an option when I was working on the development server) or edit the tables manually, which is error-prone and very uncomfortable.
So I did a little research and came across sqlalchemy-migrate and afterwards across the successor alembic.
Alembic is pretty easy to setup and with a few modifications easily integrated to flask. Run alembic init in your home directory and edit the alembic.ini. Then replace the env.py with the following and adapt it to your needs:
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from __future__ import with_statement
import os
import sys
from alembic import context
from sqlalchemy import engine_from_config, pool
from logging.config import fileConfig
sys.path.append(os.getcwd())
from adipositas import app, db
# this is the Alembic Config object, which provides
# access to the values within the .ini file in use.
config = context.config
# set the sqlalchemy url to the one defined for flask-sqlalchemy
config.set_main_option('sqlalchemy.url', app.config['SQLALCHEMY_DATABASE_URI'])
# Interpret the config file for Python logging.
# This line sets up loggers basically.
fileConfig(config.config_file_name)
# add your model's MetaData object here
# for 'autogenerate' support
# from myapp import mymodel
# target_metadata = mymodel.Base.metadata
target_metadata = db.Model.metadata
# other values from the config, defined by the needs of env.py,
# can be acquired:
# my_important_option = config.get_main_option("my_important_option")
# ... etc.
def run_migrations_offline():
"""Run migrations in 'offline' mode.
This configures the context with just a URL
and not an Engine, though an Engine is acceptable
here as well. By skipping the Engine creation
we don't even need a DBAPI to be available.
Calls to context.execute() here emit the given string to the
script output.
"""
url = config.get_main_option("sqlalchemy.url")
context.configure(url=url)
with context.begin_transaction():
context.run_migrations()
def run_migrations_online():
"""Run migrations in 'online' mode.
In this scenario we need to create an Engine
and associate a connection with the context.
"""
engine = engine_from_config(
config.get_section(config.config_ini_section),
prefix='sqlalchemy.',
poolclass=pool.NullPool)
connection = engine.connect()
context.configure(
connection=connection,
target_metadata=target_metadata
)
try:
with context.begin_transaction():
context.run_migrations()
finally:
connection.close()
if context.is_offline_mode():
run_migrations_offline()
else:
run_migrations_online()
Afterwards you can use automatic migrations for simple stuff and afterwards adjust the automatic migration to use more complex stuff.
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alembic revision -m "<your commit message here>" --autogenerate
Adapt the generated file and afterwards run:
1
alembic upgrade head
Which will then change your database. Very easy, actually. You can even perform actions on your data while migrating.
I like alembic already and I am considering to add an automatic alembic upgrade head to a git hook on the staging server, to make sure that the database layout is always up-to-date and no errors occur.
