Alembic SQLAlchemy autogenerate MetaData error

Question

I am trying to create a migration with alembic running alembic revision --autogenerate -m 'initial setup' but I'm getting the error:

FAILED: Can't proceed with --autogenerate option; environment script /Users/paul/python/my_project/alembic/env.py does not provide a MetaData object or sequence of objects to the context.

I can not figure out why I am getting this error because I have set the target_metadata in the env.py file, and added folder to the path. I tried copying the alembic setup from another one of my projects that is working without issue, but I still get the same error on this project. When I run alembic current the database gets created without issue.

My alembic.ini file:

# A generic, single database configuration.

[alembic]
# path to migration scripts
script_location = alembic

# template used to generate migration files
# file_template = %%(rev)s_%%(slug)s

# timezone to use when rendering the date
# within the migration file as well as the filename.
# string value is passed to dateutil.tz.gettz()
# leave blank for localtime
# timezone =

# max length of characters to apply to the
# "slug" field
# truncate_slug_length = 40

# set to 'true' to run the environment during
# the 'revision' command, regardless of autogenerate
# revision_environment = false

# set to 'true' to allow .pyc and .pyo files without
# a source .py file to be detected as revisions in the
# versions/ directory
# sourceless = false

# version location specification; this defaults
# to alembic/versions.  When using multiple version
# directories, initial revisions must be specified with --version-path
# version_locations = %(here)s/bar %(here)s/bat alembic/versions

# the output encoding used when revision files
# are written from script.py.mako
# output_encoding = utf-8

sqlalchemy.url = sqlite:///./src/db/data.sqlite


# Logging configuration
[loggers]
keys = root,sqlalchemy,alembic

[handlers]
keys = console

[formatters]
keys = generic

[logger_root]
level = WARN
handlers = console
qualname =

[logger_sqlalchemy]
level = WARN
handlers =
qualname = sqlalchemy.engine

[logger_alembic]
level = INFO
handlers =
qualname = alembic

[handler_console]
class = StreamHandler
args = (sys.stderr,)
level = NOTSET
formatter = generic

[formatter_generic]
format = %(levelname)-5.5s [%(name)s] %(message)s
datefmt = %H:%M:%S

My env.py file:

import sys

from logging.config import fileConfig
from pathlib import Path

from sqlalchemy import engine_from_config
from sqlalchemy import pool

from alembic import context

folder = Path(__file__).resolve().parents[1]
sys.path.insert(0, str(folder))

import src.data.__all_models
from src.data.modelbase import SqlAlchemyBase

# 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 = SqlAlchemyBase.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
    )

    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()

My modelbase.py file:

import sqlalchemy.ext.declarative as dec


SqlAlchemyBase = dec.declarative_base()

The error message shows the correct path for the env.py file so it seems to be finding the file but not seeing the target_metadata = SqlAlchemyBase.metadata line.

Answer 1

In my case the target_metadata was set to None in the migrations/env.py file.

Instead I should have used:

from src.db.models import base
target_metadata = base.metadata

with base being defined in my src.db.models file as follows:

from sqlalchemy.ext.asyncio import create_async_engine
from sqlalchemy.ext.declarative import declarative_base

base = declarative_base(bind=create_async_engine(DATABASE_URL))

Answer 2

in my case, stupidly enough, I have added my metadata correctly at the beginning of env.py but there's a line that's automatically generated after alembic initiation. the line that reads

target_metadata = None

I removed this line and used my own metadata with

from mypkg import Base
target_metadata = Base.metadata

and after this, the alembic migration worked successfully.

Answer 3

I faced the same error. The issue in my case was a malformed env.py. In particular, my database definitions were like:

# alembic.ini

[alembic]
...
databases = db1, db2
...

While the metadata definitions were misaligned like:

#env.py

target_metadata = {
    "wrong_name_1": model_db1.Base.metadata,  # Key should be "db1"
    "wrong_name_2": model_db2.Base.metadata   # Key should be "db2"
}

Answer 4

The problem turned out to be with my editor. I closed the env.py file, reopened it, and all the changes where there as expected. Then I decided to see what happened if I opened the file by command+clicking it from the error message in the console (I'm using VSCode) and none of my changes where there when I opened it this way. I tried opening the file from the side file explorer again and my changes were in the file, but alembic would still not run. I completely shut down VSCode and reopened it and after doing that the changes were in my file and alembic ran without issue.

I've never had this issue before so I have no idea what was happening with VSCode, but whatever it was restarting it solved the problem.