Alembic autogenerate does not detect existing table with mixed-case name

Question

I have an error when I want to do an auto generate migration with alembic.

Project tree :

-   alembic.ini
-   axis.py
-   tree.txt
-   
    alembic
   -   env.py
   -   README
   -   script.py.mako

       versions

# 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 = mysql+pymysql://root:**********$@localhost/amatdb_test


[post_write_hooks]
# post_write_hooks defines scripts or Python functions that are run
# on newly generated revision scripts.  See the documentation for further
# detail and examples

# format using "black" - use the console_scripts runner, against the "black" entrypoint
# hooks=black
# black.type=console_scripts
# black.entrypoint=black
# black.options=-l 79

# 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

axis.py

from sqlalchemy import create_engine
import pymysql
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String, ForeignKey, UniqueConstraint

Base = declarative_base()
engine = create_engine('mysql+pymysql://root:*******$@localhost/amatdb_test')

#The axis on which the tests are executed
class AXIS(Base):
    __tablename__ = 'Axis'
    id_axis        = Column(Integer, primary_key = True)
    name_axis      = Column(String(10), nullable=False)

env.py

from logging.config import fileConfig
from sqlalchemy import engine_from_config
from sqlalchemy import pool

from alembic import context

import sys
import os

sys.path.insert(0, os.getcwd())
print(sys.path)
from axis import Base

print(Base.metadata.sorted_tables)


# 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()

Explanation of my problem

First, I create my first auto generate migration with : alembic revision --autogenerate

The migration file seems correct.

"""Creation table

Revision ID: dcd02a3f6ff9
Revises: 
Create Date: 2020-06-03 08:47:48.198022

"""
from alembic import op
import sqlalchemy as sa


# revision identifiers, used by Alembic.
revision = 'dcd02a3f6ff9'
down_revision = None
branch_labels = None
depends_on = None


def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.create_table('Axis',
    sa.Column('id_axis', sa.Integer(), nullable=False),
    sa.Column('name_axis', sa.String(length=10), nullable=False),
    sa.PrimaryKeyConstraint('id_axis')
    )
    # ### end Alembic commands ###


def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.drop_table('Axis')
    # ### end Alembic commands ###

Then, I run : alembic upgrade head

My database is created.

So, i add a new column named "test" to my table axis.

test           = Column(String(10), default=None)

And I run a second time : alembic revision --autogenerate

I obtain this migration file

"""Ajout colonne test dans axis

Revision ID: 283ca08aec66
Revises: dcd02a3f6ff9
Create Date: 2020-06-03 08:49:19.717515

"""
from alembic import op
import sqlalchemy as sa
from sqlalchemy.dialects import mysql

# revision identifiers, used by Alembic.
revision = '283ca08aec66'
down_revision = 'dcd02a3f6ff9'
branch_labels = None
depends_on = None


def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.create_table('Axis',
    sa.Column('id_axis', sa.Integer(), nullable=False),
    sa.Column('name_axis', sa.String(length=10), nullable=False),
    sa.Column('test', sa.String(length=10), nullable=True),
    sa.PrimaryKeyConstraint('id_axis')
    )
    op.drop_table('axis')
    # ### end Alembic commands ###


def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.create_table('axis',
    sa.Column('id_axis', mysql.INTEGER(), autoincrement=True, nullable=False),
    sa.Column('name_axis', mysql.VARCHAR(length=10), nullable=False),
    sa.PrimaryKeyConstraint('id_axis'),
    mysql_collate='utf8mb4_0900_ai_ci',
    mysql_default_charset='utf8mb4',
    mysql_engine='InnoDB'
    )
    op.drop_table('Axis')
    # ### end Alembic commands ###

As you can see, alembic tries to recreate the table axis. So, when I run : alembic upgrade head

I get an error because axis already exist.

What did I miss ?

Why can't Alembic detect the existing tables ?

Thank you for your time and consideration

Answer 1

Try using a lower case table name, ie. axis instead of Axis. I'm not sure mysql even supports uppercase table names but that might be causing the problem.

An example of what might be happening, mysql probably silently lowercases the name so the first time around alembic compares Axis to nothing and goes to make Axis and it works but mysql silently creates axis. The second time alembic looks and there is no Axis, so it tries to create Axis again, mysql lower cases it and create table fails because axis already exists. Also alembic saw there was an axis, so it goes to drop that table because it is not Axis and not in the sqlalchemy schema.

Either way probably safer to just use lower case.