I am working on an application using sqlalchemy, postgres and alembic.
The project structure is as follows:
.
├── alembic.ini
├── main.py
├── migrations
│ ├── env.py
│ ├── README
│ ├── script.py.mako
│ └── versions
├── models
│ ├── base.py
│ ├── datamodel1.py
│ ├── datamodel2.py
│ └── __init__.py
└── requirements.txt
3 directories, 10 files
Where:
the content of models/base.py is :
from sqlalchemy.ext.declarative.api import declarative_base, DeclarativeMeta
Base: DeclarativeMeta = declarative_base()
The content of models/datamodel1.py is :
from models.base import Base
from sqlalchemy.sql.schema import Column
from sqlalchemy.sql.sqltypes import String, Date, Float
class Model1(Base):
__tablename__ = 'model1_table'
model1_id = Column(String, primary_key=True)
col1 = Column(String)
col2 = Column(String)
The content of models/datamodel2.py is :
from models.base import Base
from sqlalchemy.orm import relationship
from sqlalchemy.sql.sqltypes import String, Integer, Date
from sqlalchemy.sql.schema import Column, ForeignKey
# The many to may relationship table
class Model1Model2(Base):
__tablename__ = 'model1_model2_table'
id = Column(Integer, primary_key=True)
model_1_id = Column(String, ForeignKey('model1.model1_id'))
model_2_id = Column(Integer, ForeignKey('model2.model2_id'))
class Model2(Base):
__tablename__ = 'model2_table'
model2_id = Column(Integer, primary_key=True)
model2_col1 = Column(String)
model2_col2 = Column(Date)
# Many to many relationship
model1_model2 = relationship('Model1', secondary='model1_model2_table', backref='model1_table')
The content of migrations/env.py is :
from logging.config import fileConfig
from sqlalchemy import engine_from_config
from sqlalchemy import pool
from alembic import context
import sys
sys.path.append('./')
# this is the Alembic Config object, which provides
# access to the values within the .ini file in use.
config = context.config
# I added the following 2 lines to replace the sqlalchemy.url in alembic.ini file.
db_string = f'postgresql+psycopg2://{db_username}:{db_password}@{db_host}:{db_port}/{db_name}'
config.set_main_option('sqlalchemy.url', db_string)
# 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
from models.datamodel1 import Model1
from models.datamodel2 import Model2, Model1Model2
from models.base import Base
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"},
include_schemas=True,
)
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,
include_schemas=True
)
with context.begin_transaction():
context.run_migrations()
if context.is_offline_mode():
run_migrations_offline()
else:
run_migrations_online()
As for alembic.ini file I didn't make any changes, I just commented the line:
sqlalchemy.url = driver://user:pass@localhost/dbname
because I assign it in migrations/env.py
When I make changes and run alembic revision --autogenerate -m 'Add new updates' the migration files are generated correctly and everything works as expected.
But when I run alembic revision --autogenerate -m 'Add new updates' when there are no changes, it shows this in the terminal:
INFO [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO [alembic.runtime.migration] Will assume transactional DDL.
INFO [alembic.ddl.postgresql] Detected sequence named 'model2_table_model2_id_seq' as owned by integer column 'model2_table(model2_id)', assuming SERIAL and omitting
INFO [alembic.ddl.postgresql] Detected sequence named 'model1_model2_table_id_seq' as owned by integer column 'model1_model2_table(id)', assuming SERIAL and omitting
Generating /home/user/projects/dev/project/migrations/versions/45c6fbdbd23c_add_new_updates.py ... done
And it generates empty migration file that contains:
"""Add new updates
Revision ID: 45c6fbdbd23c
Revises: 5c17014a7c18
Create Date: 2021-12-27 17:11:13.964287
"""
from alembic import op
import sqlalchemy as sa
# revision identifiers, used by Alembic.
revision = '45c6fbdbd23c'
down_revision = '5c17014a7c18'
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 ###
Is this the expected behavior or it has something to do with my architecture?
How to prevent Alembic from generating those empty migration files when there are no changes?
