title: Error at migrating in Flask with sqlalchemy_utils ChoiceType
body: <p>I have a Flask model:</p>
  
  <pre><code>class User(db.Model):
      ROLE_USER = 0
      ROLE_MODERATOR = 1
      ROLE_ADMIN = 2
      ROLES = [
          (ROLE_USER, u'Regular user'),
          (ROLE_MODERATOR, u'Moderator'),
          (ROLE_ADMIN, u'Admin')
      ]
  
      id = db.Column(db.Integer, primary_key = True)
      login = db.Column(db.String(32), nullable=False, unique=True)
      first_name = db.Column(db.String(32))
      last_name = db.Column(db.String(32))
      role = db.Column(ChoiceType(ROLES), nullable=False)
  </code></pre>
  
  <p>And I've created an migration with flask-migrate (db is Postgresql):</p>
  
  <pre><code>def upgrade():
      ### commands auto generated by Alembic - please adjust! ###
      op.create_table('user',
      sa.Column('id', sa.Integer(), nullable=False),
      sa.Column('login', sa.String(length=32), nullable=False),
      sa.Column('first_name', sa.String(length=32), nullable=True),
      sa.Column('last_name', sa.String(length=32), nullable=True),
      sa.Column('role', sqlalchemy_utils.types.choice.ChoiceType(length=255), nullable=False),
      sa.PrimaryKeyConstraint('id'),
      sa.UniqueConstraint('login')
      )
  </code></pre>
  
  <p>Migration was created successfuly, but when I want to upgrade, this error rising: </p>
  
  <pre><code>TypeError: &lt;flask_script.commands.Command object at 0x7fada1e973d0&gt;: __init__() got an unexpected keyword argument 'length'
  </code></pre>
  
  <p>Can somebody explain the problem?</p>