title: How to use `migrate()` in Peewee
body: <p>I am new in peewee and I am very confused about migration. I tried the examples in the official documentation, and I got the results as follows:</p>
  <pre><code>from playhouse.migrate import *
  from peewee import *
  
  my_db = SqliteDatabase('my_database.db')
  migrator = SqliteMigrator(my_db)
  
  from peewee import *
  
  database = SqliteDatabase('my_database.db')
  
  class BaseModel(Model):
      class Meta:
          database = database
  
  class Product(BaseModel):
      name = TextField()
  
      class Meta:
          table_name = 'product'
  
  with my_db:
      my_db.create_tables([Product,])
  
  Product.create(name = 'Big Peach')
  
  price_field = IntegerField(default=0)
  migrate(migrator.add_column('product', 'price', price_field))
  db = migrator.database
  columns = db.get_columns('product')
  print(columns)
  </code></pre>
  <p>Here's what I got and the field of 'price' was added, indicating that <code>migrated ()</code> is doing well:</p>
  <blockquote>
  <p>[ColumnMetadata(name='id', data_type='INTEGER', null=False, primary_key=True, table='product', default=None),
  ColumnMetadata(name='name', data_type='TEXT', null=False, primary_key=False, table='product', default=None),
  ColumnMetadata(name='price', data_type='INTEGER', null=False, primary_key=False, table='product', default=None)]</p>
  </blockquote>
  <p>Now the problem is that when I call <code>Product.price</code>,</p>
  <pre><code>query = Product.get(Product.name == 'Big Peach')
  print(query.price)
  </code></pre>
  <p>the following result appears:</p>
  <blockquote>
  <p>'Product' object has no attribute 'price'</p>
  </blockquote>
  <p>I've tried another way to do this, in the command line:</p>
  <pre><code>python -m pwiz -e sqlite my_database.db
  </code></pre>
  <p>I got this:</p>
  <pre><code>from peewee import *
  
  database = SqliteDatabase('my_database.db')
  
  class UnknownField(object):
      def __init__(self, *_, **__): pass
  
  class BaseModel(Model):
      class Meta:
          database = database
  
  class Product(BaseModel):
      name = CharField()
      price = IntegerField()
  
      class Meta:
          table_name = 'product'   
  </code></pre>
  <p>I can use these to replace the previous Model, but there's no need to <code>migrate()</code>.
  So, my confusion is how to call the new field added by using <code>migrate()</code> in ONE <code>.py</code> file.</p>