def _group_select_pipeline(self, slice_name):
        """A pipeline that selects only the given slice's documents from the
        pipeline.
        """
        if self.group_field is None:
            return []

        name_field = self.group_field + ".name"
        return [{"$match": {"$expr": {"$eq": ["$" + name_field, slice_name]}}}]
A pipeline that selects only the given slice's documents from the
        pipeline.
        
fiftyone/core/dataset.py