langchain/vectorstores/milvus.py/Milvus/_extract_fields
class Milvus:    def _extract_fields(self) -> None:
        """Grab the existing fields from the Collection"""
        from pymilvus import Collection

        if isinstance(self.col, Collection):
            schema = self.col.schema
            for x in schema.fields:
                self.fields.append(x.name)
            # Since primary field is auto-id, no need to track it
            self.fields.remove(self._primary_field)