text: """
  Migration script to add 'purged' column to the library_dataset table.
  """
  from __future__ import print_function
  
  import logging
  
  from sqlalchemy import Boolean, Column, MetaData
  
  from galaxy.model.migrate.versions.util import add_column, drop_column, engine_false, engine_true
  
  log = logging.getLogger(__name__)
  metadata = MetaData()
  
  
  def upgrade(migrate_engine):
      print(__doc__)
      metadata.bind = migrate_engine
      metadata.reflect()
  
      c = Column('purged', Boolean, index=True, default=False)
      add_column(c, 'library_dataset', metadata, index_name='ix_library_dataset_purged')
      # Update the purged flag to the default False
      cmd = "UPDATE library_dataset SET purged = %s;" % engine_false(migrate_engine)
      try:
          migrate_engine.execute(cmd)
      except Exception:
          log.exception("Setting default data for library_dataset.purged column failed.")
  
      # Update the purged flag for those LibaryDatasets whose purged flag should be True.  This happens
      # when the LibraryDataset has no active LibraryDatasetDatasetAssociations.
      cmd = "SELECT * FROM library_dataset WHERE deleted = %s;" % engine_true(migrate_engine)
      deleted_lds = migrate_engine.execute(cmd).fetchall()
      for row in deleted_lds:
          cmd = "SELECT * FROM library_dataset_dataset_association WHERE library_dataset_id = %d AND library_dataset_dataset_association.deleted = %s;" % (int(row.id), engine_false(migrate_engine))
          active_lddas = migrate_engine.execute(cmd).fetchall()
          if not active_lddas:
              print("Updating purged column to True for LibraryDataset id : ", int(row.id))
              cmd = "UPDATE library_dataset SET purged = %s WHERE id = %d;" % (engine_true(migrate_engine), int(row.id))
              migrate_engine.execute(cmd)
  
  
  def downgrade(migrate_engine):
      metadata.bind = migrate_engine
      metadata.reflect()
  
      # SQLAlchemy Migrate has a bug when dropping a boolean column in SQLite
      if migrate_engine.name != 'sqlite':
          drop_column('purged', 'library_dataset', metadata)
  