Автогенерация Alembic не генерирует скрипт обновления

Question

Я использую sqlalchemy и postgressql в приложении Flask. Я использую инструмент миграции alembic=0.6.3. если я наберу alembic current это показывает мне:

Current revision for postgres://localhost/myDb: None

что является правильным подключением к базе данных. Но когда я бегу alembic revision --autogenerate -m 'Add user table' он генерирует шаблон alembic по умолчанию без каких-либо команд sql в нем. Подобно:

"""Add user table

Revision ID: 59b6d3503442
Revises: None
Create Date: 2015-04-06 13:42:24.540005

"""

# revision identifiers, used by Alembic.
revision = '59b6d3503442'
down_revision = None

from alembic import op
import sqlalchemy as sa

def upgrade():
    ### commands auto generated by Alembic - please adjust! ###
    pass
    ### end Alembic commands ###

def downgrade():
    ### commands auto generated by Alembic - please adjust! ###
    pass
    ### end Alembic commands ###

Я не мог найти правильное решение в SO. Вот мой env.py:

from __future__ import with_statement
from logging.config import fileConfig
import os
import sys
import warnings

from alembic import context
from sqlalchemy import create_engine, pool
from sqlalchemy.exc import SAWarning

ROOT = os.path.abspath(
    os.path.join(os.path.dirname(__file__), os.pardir, os.pardir)
)
sys.path.append(ROOT)

from myApp import Application
from myApp.extensions import db

# Don't raise exception on `SAWarning`s. For example, if Alembic does
# not recognize some column types when autogenerating migrations,
# Alembic would otherwise crash with SAWarning.
warnings.simplefilter('ignore', SAWarning)

app = Application()

# 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)

target_metadata = 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 = app.config['SQLALCHEMY_DATABASE_URI']
    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.

    """
    url = app.config['SQLALCHEMY_DATABASE_URI']
    engine = create_engine(url, poolclass=pool.NullPool)

    connection = engine.connect()
    context.configure(
        connection=connection,
        target_metadata=target_metadata
    )

    try:
        with context.begin_transaction():
            context.run_migrations()
    finally:
        connection.close()

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

а это мой alembic.ini:

# A generic, single database configuration.

[alembic]
# path to migration scripts
script_location = myApp/migrations

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

# 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

Дополнительные файлы для получения дополнительной информации

Это extensions.py файл, из которого я импортирую db за metadata:

from flask.ext.sqlalchemy import SQLAlchemy
from raven.contrib.flask import Sentry
from sqlalchemy_utils import force_instant_defaults

db = SQLAlchemy()
sentry = Sentry()
force_instant_defaults()

а это модель user.py файл:

#

 -*- coding: utf-8 -*-
from datetime import datetime
from flask.ext.login import UserMixin
from sqlalchemy_utils.types.password import PasswordType

from ..extensions import db


class User(db.Model, UserMixin):
    id = db.Column(db.Integer, primary_key=True)

    name = db.Column(
        db.Unicode(255),
        nullable=False
    )

    surname = db.Column(
        db.Unicode(255),
        nullable=False
    )

    email = db.Column(
        db.Unicode(255),
        nullable=False,
        unique=True
    )

    password = db.Column(
        PasswordType(128, schemes=['sha512_crypt']),
        nullable=False
    )

    created_at = db.Column(
        db.DateTime,
        nullable=False,
        default=datetime.utcnow
    )

    def is_active(self):
        return True

    def __repr__(self):
        return '<{cls} email={email!r}>'.format(
            cls=self.__class__.__name__,
            email=self.email,
        )

    def __str__(self):
        return unicode(self).encode('utf8')

    def __unicode__(self):
        return self.email