Dummy Pr for first good issue

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/40_knn_search.yml

@@ -283,10 +283,28 @@ setup:
   - match: {hits.total.value: 0}
 
 ---
-"Test nonexistent field":
+"Nonexistent field returns empty query":
   - requires:
-      cluster_features: "gte_v8.4.0"
-      reason: 'kNN added to search endpoint in 8.4'
+      cluster_features: "gte_v8.16.0"
+      reason: 'allow unmapped field is true'
+  - do:
+      search:
+        index: test
+        body:
+          fields: [ "name" ]
+          knn:
+            field: nonexistent
+            query_vector: [ -0.5, 90.0, -10, 14.8, -156.0 ]
+            k: 2
+            num_candidates: 3
+
+  - length: {hits.hits: 0}
+
+---
+"Nonexistent field throws error":
+  - requires:
+      cluster_features: "gte_v8.16.0"
+      reason: 'allow unmapped field is false'
   - do:
       catch: bad_request
       search:

server/src/main/java/org/elasticsearch/search/vectors/KnnVectorQueryBuilder.java

@@ -10,6 +10,7 @@
 
 import org.apache.lucene.search.BooleanClause;
 import org.apache.lucene.search.BooleanQuery;
+import org.apache.lucene.search.MatchNoDocsQuery;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.search.join.BitSetProducer;
 import org.apache.lucene.search.join.ToChildBlockJoinQuery;
@@ -436,7 +437,7 @@ protected Query doToQuery(SearchExecutionContext context) throws IOException {
             ? Math.round(Math.min(NUM_CANDS_MULTIPLICATIVE_FACTOR * requestSize, NUM_CANDS_LIMIT))
             : numCands;
         if (fieldType == null) {
-            throw new IllegalArgumentException("field [" + fieldName + "] does not exist in the mapping");
+            return new MatchNoDocsQuery();
         }
 
         if (fieldType instanceof DenseVectorFieldType == false) {

server/src/test/java/org/elasticsearch/search/vectors/AbstractKnnVectorQueryBuilderTestCase.java

@@ -10,6 +10,7 @@
 
 import org.apache.lucene.search.BooleanClause;
 import org.apache.lucene.search.BooleanQuery;
+import org.apache.lucene.search.MatchNoDocsQuery;
 import org.apache.lucene.search.Query;
 import org.elasticsearch.TransportVersion;
 import org.elasticsearch.TransportVersions;
@@ -25,6 +26,7 @@
 import org.elasticsearch.index.query.QueryBuilder;
 import org.elasticsearch.index.query.QueryBuilders;
 import org.elasticsearch.index.query.QueryRewriteContext;
+import org.elasticsearch.index.query.QueryShardException;
 import org.elasticsearch.index.query.Rewriteable;
 import org.elasticsearch.index.query.SearchExecutionContext;
 import org.elasticsearch.index.query.TermQueryBuilder;
@@ -154,13 +156,20 @@ public void testWrongDimension() {
         );
     }
 
-    public void testNonexistentField() {
+    public void testNonexistentFieldThrowsErrorWhenAllowMappingIsFalse() {
         SearchExecutionContext context = createSearchExecutionContext();
+        context.setAllowUnmappedFields(false);
         KnnVectorQueryBuilder query = new KnnVectorQueryBuilder("nonexistent", new float[] { 1.0f, 1.0f, 1.0f }, 5, 10, null);
-        IllegalArgumentException e = expectThrows(IllegalArgumentException.class, () -> query.doToQuery(context));
+        QueryShardException e = expectThrows(QueryShardException.class, () -> query.doToQuery(context));
         assertThat(e.getMessage(), containsString("field [nonexistent] does not exist in the mapping"));
     }
 
+    public void testNonexistentFieldDoesNotThrowsErrorWhenAllowMappingIsTrue() throws IOException {
+        SearchExecutionContext context = createSearchExecutionContext();
+        KnnVectorQueryBuilder query = new KnnVectorQueryBuilder("nonexistent", new float[] { 1.0f, 1.0f, 1.0f }, 5, 10, null);
+        assertThat(query.doToQuery(context), instanceOf(MatchNoDocsQuery.class));
+    }
+
     public void testWrongFieldType() {
         SearchExecutionContext context = createSearchExecutionContext();
         KnnVectorQueryBuilder query = new KnnVectorQueryBuilder(