env.py - configure and generate a SQLAlchemy engine

from __future__ import with_statement
import sys
import os.path
from alembic import context
from sqlalchemy import engine_from_config, pool
from logging.config import fileConfig
 

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. See http://alembic.zzzcomputing.com/en/latest/autogenerate.html

sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__),
                                '..')))
from models import db
target_metadata = db.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()