In the following code, I’m trying to add a plugin to the query_parser (QueryParser). The schema is already indexed and saved before the project started. However, I have an issue where the DerivationPlugin does not apply to the query_parser when using self._schema.

• self._schema is loaded from the index during initialization.
• When I create the parser using self._schema, the DerivationPlugin is
  not applied correctly during parsing.
• If I create a new schema (const_schema) with the same fields as self._schema, the DerivationPlugin works as expected.
• However, if I extract fields from self._schema and use them to create
  a new schema, the DerivationPlugin still does not work, even though
  the fields are identical.

Here is the relevant code snippet:

class BasicSearchEngine:
    def __init__(self, qdocindex, query_parser, main_field):
        self._docindex = qdocindex
        self._schema = self._docindex.get_schema()
        fields = {name: field for name, field in self._docindex.get_schema().items()}

        const_fields = {
            'a_g': NUMERIC(stored=True, unique=False),
            'a_l': NUMERIC(stored=True, unique=False),
            'a_w': NUMERIC(stored=True, unique=False),
        }
        const_schema = Schema(**const_fields)
        print(const_schema.items() == self._schema.items())  # True

        const_schema_fields = {name: field for name, field in const_schema.items()}
        print(const_schema_fields == fields)  # True

        self._parser = query_parser(main_field, self._schema, group=qparser.OrGroup)
        self._parser.add_plugin(DerivationPlugin)

Why does the DerivationPlugin not work with the parser when using self._schema, but it works when a newly created schema is used, even though the fields are identical? How can I resolve this issue?