title: Primary id sequence not incrementing
body: <p>I have a table set up like this:</p>
  
  <pre><code>USERS_TABLE = \
          Table("users", META_DATA,
                Column("id", Integer, Sequence("user_id_seq"), primary_key=True),
                Column("first_name", String(255)),
                Column("last_name", String(255))
               )
  </code></pre>
  
  <p>I am able to insert a <strong>single row</strong> like this:</p>
  
  <pre><code>INS_EXPRESSION = insert(USERS_TABLE)
  SEQ = Sequence('user_id_seq')
  INS_EXPRESSION.values(first_name="Foo", last_name="Bar")
  </code></pre>
  
  <p>But, if I try a <strong>second insert</strong>, I get this error:</p>
  
  <pre><code>sqlalchemy.exc.IntegrityError: (psycopg2.IntegrityError) null value in column "id" violates not-null constraint
  DETAIL:  Failing row contains (null, null, null).
   [SQL: 'INSERT INTO users DEFAULT VALUES']
  </code></pre>
  
  <p>How do I make the <code>user_id_seq</code> sequence <strong>auto-increment</strong>?</p>