def METHOD_NAME(self, data):
    """Get properties."""
    keys = []
    for key, value in data.items():
        if isinstance(value, dict):
            properties = value.get('properties')
            if properties:
                sub_keys = self.METHOD_NAME(properties)
                for sub_key in sub_keys:
                    if '.' in sub_key and sub_key[0] != '$':
                        keys.append('.'.join([key, sub_key]))
                    elif properties[sub_key].get('index', True):
                        keys.append('.'.join([key, sub_key]))
            elif key[0] != '$':
                keys.append(key)
    return keys