Get index for given resource.

        by default it will be `self.index`, but it can be overriden via app.config

        :param resource: resource name

def _resource_index(self, resource):
        """Get index for given resource.

        by default it will be `self.index`, but it can be overriden via app.config

        :param resource: resource name
        """
        datasource = self.get_datasource(resource)
        indexes = self._resource_config(resource, 'INDEXES') or {}
        default_index = self._resource_config(resource, 'INDEX')
        return indexes.get(datasource[0], default_index)