创建迁移脚本时没有产生initial_migration.py？

Question

我是按照《Flask web开发》第五章的内容一步一步敲的，到了数据库迁移部分，我先用python hello6.py db init创建迁移仓库，这一步shell提示完全和书上一样。然后自动创建迁移脚本的时候出了问题，如下图所示：
python hello.py db migrate -m "initial migration"

结果并没有如书中Detected added table 'roles'以及Detected added table 'users'
底下代码是hello.py

import os
from flask import Flask,render_template,session,redirect,url_for,flash
from flask.ext.script import Manager
from flask import Flask,render_template
from flask.ext.bootstrap import Bootstrap
from flask.ext.moment import Moment
from datetime import datetime
from flask.ext.wtf import Form
from wtforms import StringField, SubmitField
from wtforms.validators import Required
from flask.ext.sqlalchemy import SQLAlchemy
from flask.ext.script import Shell
from flask.ext.migrate import Migrate, MigrateCommand

basedir = os.path.abspath(os.path.dirname(__file__))

app=Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] =\
'sqlite:///' + os.path.join(basedir, 'data.sqlite')
app.config['SQLALCHEMY_COMMIT_ON_TEARDOWN'] = True
app.config['SECRET_KEY'] = 'hard to guess string'

manager=Manager(app)
bootstrap=Bootstrap(app)
moment=Moment(app)
db = SQLAlchemy(app)
migrate = Migrate(app, db)



class Role(db.Model):
    __tablename__ = 'roles'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(64), unique=True)
    users = db.relationship('User', backref='role',lazy='dynamic')

    def __repr__(self):
        return 'q<Role %r>' % self.name

class User(db.Model):
    __tablename__ = 'users'
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), unique=True, index=True)
    role_id = db.Column(db.Integer, db.ForeignKey('roles.id'))

    def __repr__(self):
        return '<User %r>' % self.username


class NameForm(Form):
    name = StringField('What is your name?', validators=[Required()])
    submit = SubmitField('Submit')

def make_shell_context():
    return dict(app=app, db=db, User=User, Role=Role)
manager.add_command("shell", Shell(make_context=make_shell_context))
manager.add_command('db', MigrateCommand)

@app.route('/', methods=['GET', 'POST'])
def index():
    form = NameForm()
    if form.validate_on_submit():
        user = User.query.filter_by(username=form.name.data).first()
        if user is None:
            user=User(username=form.name.data)
            db.session.add(user)
            session['known']=False
        else:
            session['known']=True
        session['name'] = form.name.data
        return redirect(url_for('index'))
    return render_template('index.html', 
        form=form, name=session.get('name'),
        known=session.get('known',False))

@app.route('/user/<name>')
def user(name):
    return render_template('user.html',name=name)


@app.errorhandler(404)
def page_not_found(e):
    return render_template('404.html'), 404

@app.errorhandler(500)
def internal_server_error(e):
    return render_template('500.html'), 500

if __name__=='__main__':
    manager.run()