Alembic --autogenerate producing empty migration

Question

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

Answer 1

As per @zzzeek, after I included the following in my env.py, I was able to work with --autogenerate option

in env.py under run_migrations_online()

from configuration import app
from core.expense.models import user # added my model here

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)

then I ran alembic revision --autogenerate -m "Added initial table" and got

def upgrade():
    ### commands auto generated by Alembic - please adjust! ###
    op.create_table('users',
    sa.Column('uuid', sa.GUID(), nullable=False),
    sa.Column('email', sa.String(), nullable=False),
    sa.Column('password', sa.String(), nullable=False),
    sa.Column('created_on', sa.DateTime(timezone=True), nullable=True),
    sa.Column('last_login', sa.DateTime(timezone=True), nullable=True),
    sa.PrimaryKeyConstraint('uuid'),
    sa.UniqueConstraint('email'),
    sa.UniqueConstraint('uuid')
    )
    ### end Alembic commands ###

Thank you Michael for all your help!