class ViewModel::DownMigrator

specific version requested by the client.
down migrations find a reverse path from the current schema version to the

def migrate_viewmodel!(view_name, source_version, view_hash, references) 

def migrate_viewmodel!(view_name, source_version, view_hash, references)
  path = @paths[view_name]
  return false unless path
  # In a serialized output, the source version should always be the present
  # and the current version, unless already modified by a parent migration
  required_version, current_version = @versions[view_name]
  return false if source_version == required_version
  unless source_version == current_version
    raise ViewModel::Migration::UnspecifiedVersionError.new(view_name, source_version)
  end
  path.reverse_each do |migration|
    migration.down(view_hash, references)
  end
  view_hash[ViewModel::VERSION_ATTRIBUTE] = required_version
  true
end