code: def downgrade():
      bind = op.get_bind()
      session = db.Session(bind=bind)
      for slc in session.query(Slice).all():
          if slc.datasource_type == "druid":
              slc.druid_datasource_id = slc.datasource_id
          if slc.datasource_type == "table":
              slc.table_id = slc.datasource_id
          session.commit()
      session.close()
      op.drop_column("slices", "datasource_id")
  
file_path: superset/superset/migrations/versions/2016-09-07_23-50_33d996bcc382_update_slice_model.py
imports: import sqlalchemy
  from alembic import op
  from sqlalchemy import Column, Integer, String
  from sqlalchemy.ext.declarative import declarative_base
  from superset import db
label_desc: Use image node_id 2 for calling a global function with example usage: downgrade() without return types