title: Alembic migration for GeoAlchemy2 raises NameError: name 'geoalchemy2' is not defined
body: <p>I've decided to write a small webapp using Flask, postgresql and leaflet. I wanted to store coordinates (latitude and longitude) using <a href="http://postgis.net/" rel="noreferrer">PostGIS</a> extender for postgresql. My flask application uses Flask-SQLAlchemy, blueprint and especially Flask-Migrate for the database migration process.</p>
  
  <p>Here is an excerpt of my database model:</p>
  
  <pre><code>from . import db
  from geoalchemy2 import Geometry
  
  
  class Station(db.Model):
      __tablename__ = 'stations'
  
      id = db.Column(db.Integer, primary_key=True, unique=True)
      name = db.Column(db.String(255))
      position = db.Column(Geometry('Point', srid=4326))
  </code></pre>
  
  <p>Here an excerpt of my app/<strong>init</strong>.py</p>
  
  <pre><code>import os
  from flask import Flask
  from flask_sqlalchemy import SQLAlchemy
  from flask_migrate import Migrate
  from config import config
  
  db = SQLAlchemy()
  migrate = Migrate()
  
  def create_app(config_name=None, main=True):
  
  if config_name is None:
      config_name = os.environ.get('FLASK_CONFIG', 'development')
  
  app = Flask(__name__)
  app.config.from_object(config[config_name])
  
  db.init_app(app)
  migrate.init_app(app, db)
  
  from .home import home as home_blueprint
  app.register_blueprint(home_blueprint)
  
  from .admin import admin as admin_blueprint
  app.register_blueprint(admin_blueprint, url_prefix='/admin')
  
  return app
  </code></pre>
  
  <p>Before trying to use the specific extender, I didn't have any issue to adapt my model. Since then, the migration is doing alright but the upgrade doesn't work (python manage.py db upgrade).</p>
  
  <p>Here the log I obtain from the webserver:</p>
  
  <pre><code>sa.Column('position', geoalchemy2.types.Geometry(geometry_type='POINT', srid=4326), nullable=True),
  `enter code here`NameError: name 'geoalchemy2' is not defined
  </code></pre>
  
  <p>I'm a bit lost and I didn't find much help on this specific subject. Any idea on what might cause this issue?</p>