title: Adding primary key to existing MySQL table in alembic
body: <p>I am trying to add an 'id' primary key column to an already existing MySQL table using alembic. I tried the following...</p>
  
  <pre><code>op.add_column('mytable', sa.Column('id', sa.Integer(), nullable=False))
  op.alter_column('mytable', 'id', autoincrement=True, existing_type=sa.Integer(), existing_server_default=False, existing_nullable=False) 
  </code></pre>
  
  <p>but got the following error</p>
  
  <pre><code>sqlalchemy.exc.OperationalError: (OperationalError) (1075, 'Incorrect table definition; there can be only one auto column and it must be defined as a key') 'ALTER TABLE mytable CHANGE id id INTEGER NOT NULL AUTO_INCREMENT' ()
  </code></pre>
  
  <p>looks like the sql statement generated by alembic did not add <code>PRIMARY KEY</code> at the end of the alter statement. Could I have missed some settings?</p>
  
  <p>Thanks in advance!</p>