Provide clearer error message for ES relationship filtering

optimade/filtertransformers/base_transformer.py

@@ -1,5 +1,5 @@
 import abc
-from typing import Dict
+from typing import Dict, Union
 
 from lark import Transformer, v_args
 
@@ -193,10 +193,13 @@ def not_implemented_string(self, value):
         """
         raise NotImplementedError("Comparing strings is not yet implemented.")
 
-    def property(self, args):
+    def property(self, args) -> Union[Quantity, str]:
         """property: IDENTIFIER ( "." IDENTIFIER )*"""
         quantity_name = str(args[0])
 
+        # If the quantity name matches an entry type (indicating a relationship filter)
+        # then simply return the quantity name; the inherited property
+        # must then handle any futher the nested identifiers
         if self.mapper and quantity_name in self.mapper.RELATIONSHIP_ENTRY_TYPES:
             return quantity_name
 

optimade/filtertransformers/elasticsearch.py

@@ -1,4 +1,4 @@
-from typing import Dict
+from typing import Dict, Union
 from lark import v_args
 from elasticsearch_dsl import Q, Text, Keyword, Integer, Field
 
@@ -78,7 +78,16 @@ def __init__(
 
         super().__init__(mapper=mapper)
 
-    def _field(self, quantity, nested=None):
+    def _field(self, quantity: Union[str, Quantity], nested: Quantity = None):
+
+        if (
+            isinstance(quantity, str)
+            and quantity in self.mapper.RELATIONSHIP_ENTRY_TYPES
+        ):
+            raise NotImplementedError(
+                f"Unable to filter on relationships with type {quantity!r}"
+            )
+
         if nested is not None:
             return "%s.%s" % (nested.backend_field, quantity.name)
 

tests/server/query_params/test_filter.py

@@ -149,7 +149,7 @@ def test_list_length_comparisons_aliased(check_response, check_error_response):
             request,
             expected_status=501,
             expected_title="NotImplementedError",
-            expected_detail="LENGTH is not supported for nsites",
+            expected_detail="LENGTH is not supported for 'nsites'",
         )
     else:
         check_response(request, expected_ids)
@@ -382,9 +382,9 @@ def test_filter_on_relationships(check_response, check_error_response):
     if CONFIG.database_backend == SupportedBackend.ELASTIC:
         check_error_response(
             request,
-            expected_status=400,
-            expected_title="Bad Request",
-            expected_detail="references is not a searchable quantity",
+            expected_status=501,
+            expected_title="NotImplementedError",
+            expected_detail="Unable to filter on relationships with type 'references'",
         )
         pytest.xfail("Elasticsearch backend does not support relationship filtering.")
     check_response(request, expected_ids)