title: Fetch table values using alembic and update to another table.
body: <p>I have <code>oauth secret</code> and <code>oauth key</code> in <code>client</code> table. Now I moving them to <code>oauth credentials</code> table which will be created during migration. Alembic produced following schema for upgrade. </p>
  
  <pre><code>from myapp.models import Client, ClientCredential
  from alembic import op
  import sqlalchemy as sa
  
  
  def upgrade():
  ### commands auto generated by Alembic - please adjust! ###
      op.create_table('client_credential',
      sa.Column('id', sa.Integer(), nullable=False),
      sa.Column('created_at', sa.DateTime(), nullable=False),
      sa.Column('updated_at', sa.DateTime(), nullable=False),
      sa.Column('client_id', sa.Integer(), nullable=False),
      sa.Column('key', sa.String(length=22), nullable=False),
      sa.Column('secret', sa.String(length=44), nullable=False),
      sa.Column('is_active', sa.Boolean(), nullable=False),
      sa.ForeignKeyConstraint(['client_id'], ['client.id'], ),
      sa.PrimaryKeyConstraint('id'),
      sa.UniqueConstraint('key')
     )
     # Here I need to copy data from table A to newly created Table.
     # Now Client table will not have secret and key attributes
     clients = [{'secret': client.secret, 'key': client.key, 'is_active':True, 'client_id': client.id, 'created_at': sa.func.now(), 'updated_at': sa.func.now()} for client in Client.query.all()]
     op.bulk_insert(ClientCredential, clients)
     #Also replaced above two lines with 
     #connection = op.get_bind()
     #print connection.execute(Client, Client.query.all())
     op.drop_column(u'client', u'secret')
     op.drop_column(u'client', u'key')
  </code></pre>
  
  <p>Once script enters <code>clients</code> or <code>connection.execute</code> alembic script hangs. After enabling <code>sqlalchemy logs</code>, <code>Base.Engine</code> is empty. Also tried <code>op.execute</code>, no luck.</p>
  
  <p><code>Log</code></p>
  
  <pre><code>INFO  [alembic.migration] Context impl PostgresqlImpl.
  INFO  [alembic.migration] Will assume transactional DDL.
  INFO  [alembic.migration] Running upgrade 25e7a9839cd4 -&gt; 176fb70348b9, Added  ClientCredential
  2013-09-10 23:59:08,144 INFO sqlalchemy.engine.base.Engine select version()
  INFO  [sqlalchemy.engine.base.Engine] select version()
  2013-09-10 23:59:08,145 INFO sqlalchemy.engine.base.Engine {}
  INFO  [sqlalchemy.engine.base.Engine] {}
  2013-09-10 23:59:08,146 INFO sqlalchemy.engine.base.Engine select current_schema()
  INFO  [sqlalchemy.engine.base.Engine] select current_schema()
  2013-09-10 23:59:08,146 INFO sqlalchemy.engine.base.Engine {}
  INFO  [sqlalchemy.engine.base.Engine] {}
  2013-09-10 23:59:08,148 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
  INFO  [sqlalchemy.engine.base.Engine] BEGIN (implicit)
  2013-09-10 23:59:08,150 INFO sqlalchemy.engine.base.Engine SELECT client.id AS client_id,    client.created_at AS client_created_at, client.updated_at AS client_updated_at, client.user_id   AS client_user_id, client.org_id AS client_org_id, client.title AS client_title,   client.description AS client_description, client.website AS client_website, client.redirect_uri AS client_redirect_uri, client.notification_uri AS   client_notification_uri, client.iframe_uri AS client_iframe_uri, client.resource_uri AS client_resource_uri, client.active AS client_active, client.allow_any_login AS client_allow_any_login, client.team_access AS client_team_access, client.trusted AS client_trusted
  FROM client
  INFO  [sqlalchemy.engine.base.Engine] SELECT client.id AS client_id, client.created_at AS    client_created_at, client.updated_at AS client_updated_at, client.user_id AS client_user_id, client.org_id AS client_org_id, client.title AS client_title, client.description AS client_description, client.website AS client_website, client.redirect_uri AS client_redirect_uri, client.notification_uri AS client_notification_uri, client.iframe_uri AS client_iframe_uri, client.resource_uri AS client_resource_uri, client.active AS client_active, client.allow_any_login AS client_allow_any_login, client.team_access AS client_team_access, client.trusted AS client_trusted
  FROM client
  2013-09-10 23:59:08,150 INFO sqlalchemy.engine.base.Engine {}
  INFO  [sqlalchemy.engine.base.Engine] {}
  </code></pre>
  
  <p>How can I copy the values from <code>client</code> table to <code>client_credential</code> table using alembic migration? </p>