code: def get_nested_fields(schema, model_field=False):
      
  
      nested_fields = []
      for (key, value) in schema._declared_fields.items():
          if isinstance(value, List) and isinstance(value.container, Nested):
              nested_fields.append(key)
          elif isinstance(value, Nested):
              nested_fields.append(key)
  
      if model_field is True:
          nested_fields = [get_model_field(schema, key) for key in nested_fields]
  
      return nested_fields
docs: Return nested fields of a schema to support a join
  
      :param Schema schema: a marshmallow schema
      :param boolean model_field: whether to extract the model field for the nested fields
      :return list: list of nested fields of the schema