title: sqlalchemy and auto increments with postgresql
body: <p>I created a table with a primary key and a sequence but via the debug ad later looking at the table design, the sequence isn't applied, just created.</p>
  
  <pre><code>from sqlalchemy import create_engine, MetaData, Table, Column,Integer,String,Boolean,Sequence
  from sqlalchemy.orm import mapper, sessionmaker
  from sqlalchemy.ext.declarative import declarative_base
  import json
  class Bookmarks(object):
      pass
  
  #----------------------------------------------------------------------
  engine = create_engine('postgresql://iser:p@host/sconf', echo=True)
  Base = declarative_base()
  
  class Tramo(Base):
      __tablename__ = 'tramos'
      __mapper_args__ = {'column_prefix':'tramos'}
  
      id = Column(Integer, Sequence('seq_tramos_id', start=1, increment=1),primary_key=True)
      nombre = Column(String)
      tramo_data = Column(String)
      estado = Column(Boolean,default=True)
  
      def __init__(self,nombre,tramo_data):
          self.nombre=nombre
          self.tramo_data=tramo_data
  
      def __repr__(self):
          return '[id:%d][nombre:%s][tramo:%s]' % self.id, self.nombre,self.tramo_data
  
  Session = sessionmaker(bind=engine)
  session = Session()
  
  tabla = Tramo.__table__
  metadata = Base.metadata
  metadata.create_all(engine)
  </code></pre>
  
  <p>the table is just created like this</p>
  
  <pre><code>CREATE TABLE tramos (
      id INTEGER NOT NULL, 
      nombre VARCHAR, 
      tramo_data VARCHAR, 
      estado BOOLEAN, 
      PRIMARY KEY (id)
  )
  </code></pre>
  
  <p>I was hoping to see the declartion of the default nexval of the sequence 
  but it isn't there.</p>
  
  <p>I also used the <code>__mapper_args__</code> but looks like it's been ignored.</p>
  
  <p>Am I missing something?</p>