Integrating Alembic with SQLAlchemy - sqlalchemy

Question

I'm looking at a way to integrate Alembic with SQLAlchemy. What I need is a way so that Alembic detects any changes I make in models.py automatically and updates it in the MySQL database when I run alembic revision -m "<message_here>" and alembic upgrade head.
Here is what I have at the moment.
This is my application directory structure.
/myapplication
models.py
__init__.py
app.py
/migrations
env.py
script.py.mako
/versions
The models.py contains the following.
from sqlalchemy import Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class User(Base):
""" `User` stores the basic info about a user
"""
__tablename__ = 'user'
id = Column(Integer, primary_key=True)
name = Column(String(255), nullable=False)
phone = Column(String(15), nullable=False)
I have configured my alembic.ini with my database credentials at sqlalchemy.url
I have the following in my env.py
from __future__ import with_statement
from alembic import context
from sqlalchemy import engine_from_config, pool
from logging.config import fileConfig
from myapplication import models
# this is the Alembic Config object, which provides
# access to the values within the .ini file in use.
config = context.config
# 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 = models.Base.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()
But when the run the migration using
alembic revision -m "initial"
It says
Generating
migrations/versions/2d9d8de1aa80_initial.py...done
But when I open the migrations/versions/9aa5864e4c8_initial.py, this is what I see.
"""initial
Revision ID: 2d9d8de1aa80
Revises: None
Create Date: 2013-02-23 12:21:52.389906
"""
# revision identifiers, used by Alembic.
revision = '2d9d8de1aa80'
down_revision = None
from alembic import op
import sqlalchemy as sa
def upgrade():
pass
def downgrade():
pass
The version file has nothing about the tables I was expecting it to create and later sync with MySQL when I run the command alembic upgrade head. How do I configure Alembic so that when I run the alembic revision command, It picks the schema from models.py and generates a version file? I think I am missing something trivial here. Not sure what it is!

Answer 1

You want to run the revision subcommand with the --autogenerate flag so it inspects the models for changes.
alembic revision --autogenerate -m "some message"
Make sure you're aware of the limitations of the autogenerate option.