title: db.create_all() doesn't create tables defined in separate file
body: <p>I am trying to create the tables for my models, which are defined in a separate module from my app.  I call <code>db.create_all()</code>, but no tables are created and there are no errors.  I've defined the models and <a href="https://stackoverflow.com/questions/20744277/sqlalchemy-create-all-does-not-create-tables/20749534#20749534">imported them before calling <code>create_all</code></a>.  Why doesn't this work?</p>
  
  <pre><code>from flask import Flask
  from flask.ext.sqlalchemy import SQLAlchemy
  
  app = Flask(__name__)
  app.config['SQLALCHEMY_DATABASE_URI'] = 'postgresql+psycopg2://postgres:123@localhost/flask'
  db = SQLAlchemy(app)
  
  from models import User
  
  db.create_all()
  db.session.commit()
  </code></pre>
  
  <p><code>models.py</code>:</p>
  
  <pre><code>from flask.ext.sqlalchemy import SQLAlchemy
  
  db = SQLAlchemy()
  
  class User(db.Model):
    __tablename__ = 'users'
    uid = db.Column(db.Integer, primary_key = True)
  </code></pre>