[Python] Alembic --autogenerate producing empty migration

I am trying to use Alembic for the first time and want to use --autogenerate feature described here

My project structure looks like

project/
configuration/
__init__.py
dev.py
test.py
core/
app/
models/
__init__.py
user.py
db/
alembic/
versions/
env.py
alembic.ini


I am using Flask and SQLAlchemy and their Flask-SQLAlchemy extension. my model User looks like

class User(UserMixin, db.Model):
__tablename__ = 'users'
# noinspection PyShadowingBuiltins
uuid = Column('uuid', GUID(), default=uuid.uuid4, primary_key=True,
unique=True)
email = Column('email', String, nullable=False, unique=True)
_password = Column('password', String, nullable=False)
created_on = Column('created_on', sa.types.DateTime(timezone=True),
default=datetime.utcnow())
last_login = Column('last_login', sa.types.DateTime(timezone=True),
onupdate=datetime.utcnow())


As described here, I modified env.py to look like

from configuration import app

alembic_config = config.get_section(config.config_ini_section)
alembic_config['sqlalchemy.url'] = app.config['SQLALCHEMY_DATABASE_URI']
engine = engine_from_config(
alembic_config,
prefix='sqlalchemy.',
poolclass=pool.NullPool)


and

from configuration import db


target_metadata = db.metadata


where configuration.__init__py looks like

from flask import Flask
from flask.ext.sqlalchemy import SQLAlchemy
import dev


app = Flask(__name__)
app.config.from_envvar('SETTINGS_PT')
db = SQLAlchemy(app)


Now when I run migration

$alembic revision --autogenerate -m "Added user table"
INFO [alembic.migration] Context impl PostgresqlImpl.
INFO [alembic.migration] Will assume transactional DDL.
Generating /Users/me/IdeaProjects/project/db/alembic/versions/55a9d5
35d8ae_added_user_table.py...done


but file alembic/versions/55a9d5 has empty upgrade() and downgrade() methods

"""Added user table

Revision ID: 1b62a62eef0d
Revises: None
Create Date: 2013-03-27 06:37:08.314177

"""

# revision identifiers, used by Alembic.
revision = '1b62a62eef0d'
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 ###


How come it is not able to understand that there is a new User model? Please help

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