text: from __future__ import with_statement
  from flask import current_app
  from alembic import context
  from sqlalchemy import engine_from_config, pool
  from logging.config import fileConfig
  import logging
  
  # 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')
  
  # 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',
                         current_app.config.get('SQLALCHEMY_DATABASE_URI'))
  target_metadata = current_app.extensions['migrate'].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)
  
      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.')
  
      engine = engine_from_config(config.get_section(config.config_ini_section),
                                  prefix='sqlalchemy.',
                                  poolclass=pool.NullPool)
  
      connection = engine.connect()
      context.configure(connection=connection,
                        target_metadata=target_metadata,
                        process_revision_directives=process_revision_directives,
                        **current_app.extensions['migrate'].configure_args)
  
      try:
          with context.begin_transaction():
              context.run_migrations()
      finally:
          connection.close()
  
  
  if context.is_offline_mode():
      run_migrations_offline()
  else:
      run_migrations_online()
  