From 651bc39565442c45dee6639dc961e214dce9f4d3 Mon Sep 17 00:00:00 2001
From: Mike Pellegrini <mike.pellegrini@elastic.co>
Date: Tue, 24 Jun 2025 19:25:24 -0400
Subject: [PATCH] Simplified Linear & RRF Retrievers - Return error on empty
 fields param (#129962)

---
 docs/changelog/129962.yaml                      |  5 +++++
 .../rank/MultiFieldsInnerRetrieverUtils.java    | 17 +++++++++++++----
 .../rank/linear/LinearRetrieverBuilder.java     |  4 ++--
 .../xpack/rank/rrf/RRFRetrieverBuilder.java     |  4 ++--
 .../linear/20_linear_retriever_simplified.yml   | 12 ++++++++++++
 .../test/rrf/310_rrf_retriever_simplified.yml   | 12 ++++++++++++
 6 files changed, 46 insertions(+), 8 deletions(-)
 create mode 100644 docs/changelog/129962.yaml

diff --git a/docs/changelog/129962.yaml b/docs/changelog/129962.yaml
new file mode 100644
index 000000000000..dd06742a7479
--- /dev/null
+++ b/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: []
diff --git a/x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/MultiFieldsInnerRetrieverUtils.java b/x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/MultiFieldsInnerRetrieverUtils.java
index 0715b4fa6754..8aa5dbf366a7 100644
--- a/x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/MultiFieldsInnerRetrieverUtils.java
+++ b/x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/MultiFieldsInnerRetrieverUtils.java
@@ -56,7 +56,7 @@ public class MultiFieldsInnerRetrieverUtils {
      */
     public static ActionRequestValidationException validateParams(
         List<CompoundRetrieverBuilder.RetrieverSource> innerRetrievers,
-        List<String> fields,
+        @Nullable List<String> fields,
         @Nullable String query,
         String retrieverName,
         String retrieversParamName,
@@ -64,7 +64,7 @@ public class MultiFieldsInnerRetrieverUtils {
         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 class MultiFieldsInnerRetrieverUtils {
                 );
             }
 
+            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 class MultiFieldsInnerRetrieverUtils {
      * @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);
         }
diff --git a/x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/linear/LinearRetrieverBuilder.java b/x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/linear/LinearRetrieverBuilder.java
index f0c36f9819af..c1a3f7d17448 100644
--- a/x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/linear/LinearRetrieverBuilder.java
+++ b/x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/linear/LinearRetrieverBuilder.java
@@ -163,7 +163,7 @@ public final class LinearRetrieverBuilder extends CompoundRetrieverBuilder<Linea
             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 final class LinearRetrieverBuilder extends CompoundRetrieverBuilder<Linea
             builder.endArray();
         }
 
-        if (fields.isEmpty() == false) {
+        if (fields != null) {
             builder.startArray(FIELDS_FIELD.getPreferredName());
             for (String field : fields) {
                 builder.value(field);
diff --git a/x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/rrf/RRFRetrieverBuilder.java b/x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/rrf/RRFRetrieverBuilder.java
index 9fb7fdea21bb..c05fd3bd0a11 100644
--- a/x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/rrf/RRFRetrieverBuilder.java
+++ b/x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/rrf/RRFRetrieverBuilder.java
@@ -115,7 +115,7 @@ public final class RRFRetrieverBuilder extends CompoundRetrieverBuilder<RRFRetri
     ) {
         // 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 final class RRFRetrieverBuilder extends CompoundRetrieverBuilder<RRFRetri
             builder.endArray();
         }
 
-        if (fields.isEmpty() == false) {
+        if (fields != null) {
             builder.startArray(FIELDS_FIELD.getPreferredName());
             for (String field : fields) {
                 builder.value(field);
diff --git a/x-pack/plugin/rank-rrf/src/yamlRestTest/resources/rest-api-spec/test/linear/20_linear_retriever_simplified.yml b/x-pack/plugin/rank-rrf/src/yamlRestTest/resources/rest-api-spec/test/linear/20_linear_retriever_simplified.yml
index dea4608c13dd..7ab0d727a738 100644
--- a/x-pack/plugin/rank-rrf/src/yamlRestTest/resources/rest-api-spec/test/linear/20_linear_retriever_simplified.yml
+++ b/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:
diff --git a/x-pack/plugin/rank-rrf/src/yamlRestTest/resources/rest-api-spec/test/rrf/310_rrf_retriever_simplified.yml b/x-pack/plugin/rank-rrf/src/yamlRestTest/resources/rest-api-spec/test/rrf/310_rrf_retriever_simplified.yml
index 0fd15a0ad2b7..cd0328069192 100644
--- a/x-pack/plugin/rank-rrf/src/yamlRestTest/resources/rest-api-spec/test/rrf/310_rrf_retriever_simplified.yml
+++ b/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: