shopdb-flask/migrations/env.py

import logging
from logging.config import fileConfig

from flask import current_app

from alembic import context

# Registers the compiler hook that forces utf8mb4 + DYNAMIC on MySQL. Imported
# for the side effect, and shared with the PLUGIN chains via
# shopdb/plugins/alembic_template.py - it used to live here, so it covered core
# only and plugin baselines inherited the server default charset.
import shopdb.utils.mysql_charset  # noqa: F401

# 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)
logger = logging.getLogger('alembic.env')


def get_engine():
    try:
        # this works with Flask-SQLAlchemy<3 and Alchemical
        return current_app.extensions['migrate'].db.get_engine()
    except (TypeError, AttributeError):
        # this works with Flask-SQLAlchemy>=3
        return current_app.extensions['migrate'].db.engine


def get_engine_url():
    try:
        return get_engine().url.render_as_string(hide_password=False).replace(
            '%', '%%')
    except AttributeError:
        return str(get_engine().url).replace('%', '%%')


# add your model's MetaData object here
# for 'autogenerate' support
# from myapp import mymodel
# target_metadata = mymodel.Base.metadata
config.set_main_option('sqlalchemy.url', get_engine_url())
target_db = current_app.extensions['migrate'].db

# 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 get_metadata():
    if hasattr(target_db, 'metadatas'):
        return target_db.metadatas[None]
    return target_db.metadata


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=get_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.

    """

    # this callback is used to prevent an auto-migration from being generated
    # when there are no changes to the schema
    # reference: http://alembic.zzzcomputing.com/en/latest/cookbook.html
    def process_revision_directives(context, revision, directives):
        if getattr(config.cmd_opts, 'autogenerate', False):
            script = directives[0]
            if script.upgrade_ops.is_empty():
                directives[:] = []
                logger.info('No changes in schema detected.')

    conf_args = current_app.extensions['migrate'].configure_args
    if conf_args.get("process_revision_directives") is None:
        conf_args["process_revision_directives"] = process_revision_directives

    connectable = get_engine()

    # Alembic creates alembic_version.version_num as VARCHAR(32), but some
    # revision ids in this chain exceed 32 chars. On MySQL that truncates the
    # stored id, so the next migration's version bump matches 0 rows and the
    # upgrade dies. Force the column wide in its OWN committed connection first
    # (keeping it out of alembic's migration transaction), so long ids never
    # truncate - fresh or existing.
    try:
        with connectable.connect() as prep:
            prep.exec_driver_sql(
                'CREATE TABLE IF NOT EXISTS alembic_version '
                '(version_num VARCHAR(128) NOT NULL, '
                'CONSTRAINT alembic_version_pkc PRIMARY KEY (version_num))'
            )
            prep.exec_driver_sql(
                'ALTER TABLE alembic_version MODIFY version_num VARCHAR(128) NOT NULL'
            )
            prep.commit()
    except Exception:
        # Non-MySQL backends (e.g. sqlite in tests) - alembic's default is fine.
        pass

    with connectable.connect() as connection:
        # Relax the session sql_mode for the migration run. Some historical
        # migrations seed reference rows with raw INSERTs that omit NOT-NULL
        # timestamp columns (the ORM supplies those via Python defaults at
        # runtime, but a raw migration INSERT does not). MySQL 5.x's lax default
        # accepted that; strict MySQL 8 rejects it with 1364 "Field 'createddate'
        # doesn't have a default value". Dropping STRICT_TRANS_TABLES for the
        # migration session only (the app's own connections keep their mode)
        # makes the chain portable across MySQL versions.
        try:
            connection.exec_driver_sql(
                "SET SESSION sql_mode = 'NO_ENGINE_SUBSTITUTION'")
        except Exception:
            pass  # non-MySQL backends (e.g. sqlite in tests)

        context.configure(
            connection=connection,
            target_metadata=get_metadata(),
            **conf_args
        )

        with context.begin_transaction():
            context.run_migrations()

        # COMMIT THE RUN. SQLAlchemy 2.0 connections do not autocommit, and on
        # MySQL alembic reports "non-transactional DDL" so begin_transaction()
        # above is a no-op - nothing here commits on its own.
        #
        # It looked like it worked because MySQL implicitly commits on DDL: each
        # ALTER/CREATE flushed everything queued before it, including the
        # PREVIOUS migration's version stamp. The LAST migration of every run
        # has no DDL after it, so its stamp was rolled back at close. The column
        # changes survived (already committed by their own DDL) while
        # alembic_version stayed one revision behind, so `flask db upgrade`
        # exited 0 having silently re-run the final migration, and re-ran it
        # again on the next deploy. A migration that is not idempotent would
        # apply twice.
        connection.commit()


if context.is_offline_mode():
    run_migrations_offline()
else:
    run_migrations_online()