<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, table_name, primary_key, instance):
 """ replaces document identified by the primary_key or creates one if a matching document does not exist"""

        assert isinstance(primary_key, dict)
        assert isinstance(instance, BaseDocument)
        collection = self._db[table_name]

        # work with a copy of the document, as the direct type change of the _id field
        # is later negated by the `BaseDocument.to_json` method
        document = instance.document
        if '_id' in document:
            document['_id'] = ObjectId(document['_id'])

        update_result = collection.replace_one(filter=primary_key, replacement=document, upsert=True)
        if update_result.upserted_id:
            instance['_id'] = update_result.upserted_id
        return update_result.upserted_id