I followed the documentation from Alembic to auto-generate migrations. My project structure looks like this:
alembic/
versions/
env.py
README
script.py.mako
data/
__init__.py
db.py
models.py
alembic.ini
app.db
I made changes to env.py by exactly following the document:
from logging.config import fileConfig
from sqlalchemy import engine_from_config
from sqlalchemy import pool
from alembic import context
from data.models import Base
# 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 = 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,
target_metadata=target_metadata,
literal_binds=True,
dialect_opts={"paramstyle": "named"},
)
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.
"""
connectable = engine_from_config(
config.get_section(config.config_ini_section),
prefix="sqlalchemy.",
poolclass=pool.NullPool,
)
with connectable.connect() as connection:
context.configure(
connection=connection, target_metadata=target_metadata
)
with context.begin_transaction():
context.run_migrations()
if context.is_offline_mode():
run_migrations_offline()
else:
run_migrations_online()
This is my __init__.py:
import os
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from .models import Base
basedir = os.path.abspath(os.path.dirname(os.path.dirname(__file__)))
db_url = os.environ.get('DATABASE_URL') or \
'sqlite:///' + os.path.join(basedir, 'app.db')
engine = create_engine(db_url, echo=False)
Base.metadata.create_all(engine)
session = sessionmaker(bind=engine)
I created a User class in models.py like this:
from sqlalchemy import Column, Sequence, Integer, String
from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()
class User(Base):
__tablename__ = 'user'
id = Column(Integer, Sequence('id_seq'), primary_key=True)
first_name = Column(String, nullable=False)
last_name = Column(String, nullable=False)
def __repr__(self):
return "<%s('%s', '%s')>" \
% (self.__class__.__qualname__, self.first_name,
self.last_name)
After that, I run migration by:
alembic revision --autogenerate -m "Added user table"
However, I got empty upgrade() and downgrade() in migration file:
"""Added user table
Revision ID: 279933caec54
Revises:
Create Date: 2021-05-12 16:21:05.772568
"""
from alembic import op
import sqlalchemy as sa
# revision identifiers, used by Alembic.
revision = '279933caec54'
down_revision = None
branch_labels = None
depends_on = None
def upgrade():
# ### commands auto generated by Alembic - please adjust! ###
pass
# ### end Alembic commands ###
def downgrade():
# ### commands auto generated by Alembic - please adjust! ###
pass
# ### end Alembic commands ###
But when I checked the database, the user table was created there (I didn't do it myself). How did it happen? How did alembic create empty migration but generating the table?
I found the answer in this question. The problem described in this question is not the same as mine (the alembic didn't create an empty migration file in this case) though. That's why I asked the question here (may be seen as duplicated).
So as the answer suggested, I commented the Base.metadata.create_all(engine) in the __init__.py. It is this line of code that creates the user table (if it not exist). So when alembic checks my database, the table is already there. Since no differences are found, the upgrade() and downgrade() are empty.