A custom property transformer to translate a ndb.JsonProperty into a repeated record with fields for each of the keys in the original JSON.

custom_bq_transformers.py

class TransformedVideoTranslationInfo(bq_property_transform.TransformedEntity):

    CUSTOM_SCHEMAS = {
        'translated_youtube_ids': {
            'name': 'translated_youtube_ids',
            'type': 'record',
            'mode': 'repeated',
            'fields': [
                {'name': 'language',
                 'type': 'string'},
                {'name': 'youtube_id',
                 'type': 'string'}
            ]
        }
    }

    @classmethod
    def transform_custom_property(cls, entity, schema_field):
        """Do a custom transform for the translated_youtube_ids field.

        This will convert the json property to a repeated record property
        containing subproperties 'language' and 'youtube_id' for each
        translation.
        """
        if schema_field['name'] == 'translated_youtube_ids':
            languages = []
            parsed_json = json.loads(entity[schema_field['name']])
            for k, v in parsed_json.items():
                languages.append({'language': k, 'youtube_id': v})
            return {'translated_youtube_ids': languages}