<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def update(self, statement):
 """ Modifies an entry in the database. Creates an entry if one does not exist. """

        Statement = self.get_model('statement')
        Tag = self.get_model('tag')

        if statement is not None:
            session = self.Session()
            record = None

            if hasattr(statement, 'id') and statement.id is not None:
                record = session.query(Statement).get(statement.id)
            else:
                record = session.query(Statement).filter(
                    Statement.text == statement.text,
                    Statement.conversation == statement.conversation,
                ).first()

                # Create a new statement entry if one does not already exist
                if not record:
                    record = Statement(
                        text=statement.text,
                        conversation=statement.conversation,
                        persona=statement.persona
                    )

            # Update the response value
            record.in_response_to = statement.in_response_to

            record.created_at = statement.created_at

            record.search_text = self.tagger.get_bigram_pair_string(statement.text)

            if statement.in_response_to:
                record.search_in_response_to = self.tagger.get_bigram_pair_string(statement.in_response_to)

            for tag_name in statement.get_tags():
                tag = session.query(Tag).filter_by(name=tag_name).first()

                if not tag:
                    # Create the record
                    tag = Tag(name=tag_name)

                record.tags.append(tag)

            session.add(record)

            self._session_finish(session)