new_contents: from datetime import datetime
  from sqlalchemy import Column, ForeignKey, MetaData, Table
  from sqlalchemy import DateTime, Integer, Unicode, UnicodeText
  
  
  meta = MetaData()
  
  milestone_table = Table(
      'milestone', meta,
      Column('id', Integer, primary_key=True),
      Column('instance_id', Integer, ForeignKey('instance.id'), nullable=False),
      Column('creator_id', Integer, ForeignKey('user.id'), nullable=False),
      Column('title', Unicode(255), nullable=True),
      Column('text', UnicodeText(), nullable=True),
      Column('time', DateTime),
      Column('create_time', DateTime, default=datetime.utcnow),
      Column('delete_time', DateTime)
  )
  
  
  def upgrade(migrate_engine):
      meta.bind = migrate_engine
  
      Table('user', meta, autoload=True)
      Table('instance', meta, autoload=True)
      Table('badge', meta, autoload=True)
      category_column = Column('category_id', Integer, ForeignKey('badge.id'),
                               nullable=True)
      category_column.create(milestone_table)
  
  
  def downgrade(migrate_engine):
      raise NotImplementedError()
  
subject: Add migration step