Simplified Linear & RRF Retrievers - Return error on empty fields param

docs/changelog/129962.yaml

@@ -0,0 +1,5 @@
+pr: 129962
+summary: Simplified Linear & RRF Retrievers - Return error on empty fields param
+area: Search
+type: bug
+issues: []

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/MultiFieldsInnerRetrieverUtils.java

@@ -56,15 +56,15 @@ public record WeightedRetrieverSource(CompoundRetrieverBuilder.RetrieverSource r
  */
  public static ActionRequestValidationException validateParams(
  List<CompoundRetrieverBuilder.RetrieverSource> innerRetrievers,
- List<String> fields,
+ @Nullable List<String> fields,
  @Nullable String query,
  String retrieverName,
  String retrieversParamName,
  String fieldsParamName,
  String queryParamName,
  ActionRequestValidationException validationException
  ) {
- if (fields.isEmpty() == false || query != null) {
+ if (fields != null || query != null) {
  // Using the multi-fields query format
  if (query == null) {
  // Return early here because the following validation checks assume a query param value is provided
@@ -87,6 +87,13 @@ public static ActionRequestValidationException validateParams(
  );
  }
 
+ if (fields != null && fields.isEmpty()) {
+ validationException = addValidationError(
+ String.format(Locale.ROOT, "[%s] [%s] cannot be empty", retrieverName, fieldsParamName),
+ validationException
+ );
+ }
+
  if (innerRetrievers.isEmpty() == false) {
  validationException = addValidationError(
  String.format(Locale.ROOT, "[%s] cannot combine [%s] and [%s]", retrieverName, retrieversParamName, queryParamName),
@@ -131,13 +138,15 @@ public static ActionRequestValidationException validateParams(
  * @return The inner retriever tree as a {@code RetrieverBuilder} list
  */
  public static List<RetrieverBuilder> generateInnerRetrievers(
- List<String> fieldsAndWeights,
+ @Nullable List<String> fieldsAndWeights,
  String query,
  Collection<IndexMetadata> indicesMetadata,
  Function<List<WeightedRetrieverSource>, CompoundRetrieverBuilder<?>> innerNormalizerGenerator,
  @Nullable Consumer<Float> weightValidator
  ) {
- Map<String, Float> parsedFieldsAndWeights = QueryParserHelper.parseFieldsAndWeights(fieldsAndWeights);
+ Map<String, Float> parsedFieldsAndWeights = fieldsAndWeights != null
+ ? QueryParserHelper.parseFieldsAndWeights(fieldsAndWeights)
+ : Map.of();
  if (weightValidator != null) {
  parsedFieldsAndWeights.values().forEach(weightValidator);
  }

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/linear/LinearRetrieverBuilder.java

@@ -163,7 +163,7 @@ public LinearRetrieverBuilder(
  throw new IllegalArgumentException("The number of normalizers must match the number of inner retrievers");
  }
 
- this.fields = fields == null ? List.of() : List.copyOf(fields);
+ this.fields = fields == null ? null : List.copyOf(fields);
  this.query = query;
  this.normalizer = normalizer;
  this.weights = weights;
@@ -400,7 +400,7 @@ public void doToXContent(XContentBuilder builder, Params params) throws IOExcept
  builder.endArray();
  }
 
- if (fields.isEmpty() == false) {
+ if (fields != null) {
  builder.startArray(FIELDS_FIELD.getPreferredName());
  for (String field : fields) {
  builder.value(field);

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/rrf/RRFRetrieverBuilder.java

@@ -115,7 +115,7 @@ public RRFRetrieverBuilder(
  ) {
  // Use a mutable list for childRetrievers so that we can use addChild
  super(childRetrievers == null ? new ArrayList<>() : new ArrayList<>(childRetrievers), rankWindowSize);
- this.fields = fields == null ? List.of() : List.copyOf(fields);
+ this.fields = fields == null ? null : List.copyOf(fields);
  this.query = query;
  this.rankConstant = rankConstant;
  }
@@ -293,7 +293,7 @@ public void doToXContent(XContentBuilder builder, Params params) throws IOExcept
  builder.endArray();
  }
 
- if (fields.isEmpty() == false) {
+ if (fields != null) {
  builder.startArray(FIELDS_FIELD.getPreferredName());
  for (String field : fields) {
  builder.value(field);

x-pack/plugin/rank-rrf/src/yamlRestTest/resources/rest-api-spec/test/linear/20_linear_retriever_simplified.yml

@@ -465,6 +465,18 @@ setup:
 
  - contains: { error.root_cause.0.reason: "[linear] [query] cannot be empty" }
 
+ - do:
+ catch: bad_request
+ search:
+ index: test-index
+ body:
+ retriever:
+ linear:
+ fields: [ ]
+ query: "foo"
+
+ - contains: { error.root_cause.0.reason: "[linear] [fields] cannot be empty" }
+
  - do:
  catch: bad_request
  search:

x-pack/plugin/rank-rrf/src/yamlRestTest/resources/rest-api-spec/test/rrf/310_rrf_retriever_simplified.yml

@@ -359,6 +359,18 @@ setup:
 
  - contains: { error.root_cause.0.reason: "[rrf] [query] cannot be empty" }
 
+ - do:
+ catch: bad_request
+ search:
+ index: test-index
+ body:
+ retriever:
+ rrf:
+ fields: [ ]
+ query: "foo"
+
+ - contains: { error.root_cause.0.reason: "[rrf] [fields] cannot be empty" }
+
  - do:
  catch: bad_request
  search: