def __init__(self, importer, old_event, event):
        self.importer = importer
        self.old_event = old_event
        self.event = event
        self.event_person_map = {}
        self.legacy_session_map = {}
        self.legacy_session_ids_used = set()
        self.legacy_contribution_map = {}
        self.legacy_contribution_type_map = {}
        self.legacy_contribution_field_map = {}
        self.legacy_field_option_id_map = {}
        # we know some relationships are empty; prevent SA from loading them
        set_committed_value(self.event, 'references', [])
        set_committed_value(self.event, 'person_links', [])