Fix LTR rescorer with model alias

docs/changelog/126273.yaml

@@ -0,0 +1,5 @@
+pr: 126273
+summary: Fix LTR rescorer with model alias
+area: Ranking
+type: bug
+issues: []

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/loadingservice/ModelLoadingService.java

@@ -239,8 +239,7 @@ public ModelLoadingService(
         this.licenseState = licenseState;
     }
 
-    // for testing
-    String getModelId(String modelIdOrAlias) {
+    public String getModelId(String modelIdOrAlias) {
         return modelAliasToId.getOrDefault(modelIdOrAlias, modelIdOrAlias);
     }
 

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/inference/ltr/LearningToRankService.java

@@ -101,7 +101,7 @@ public void loadLocalModel(String modelId, ActionListener<LocalModel> listener)
      */
     public void loadLearningToRankConfig(String modelId, Map<String, Object> params, ActionListener<LearningToRankConfig> listener) {
         trainedModelProvider.getTrainedModel(
-            modelId,
+            modelLoadingService.getModelId(modelId),
             GetTrainedModelsAction.Includes.all(),
             null,
             ActionListener.wrap(trainedModelConfig -> {

x-pack/plugin/ml/src/test/java/org/elasticsearch/xpack/ml/inference/ltr/LearningToRankServiceTests.java

@@ -42,12 +42,14 @@
 import static org.hamcrest.Matchers.hasKey;
 import static org.hamcrest.Matchers.hasSize;
 import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyString;
 import static org.mockito.ArgumentMatchers.argThat;
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.ArgumentMatchers.isA;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
 
 public class LearningToRankServiceTests extends ESTestCase {
     public static final String GOOD_MODEL = "inference-entity-id";
@@ -185,7 +187,10 @@ protected NamedXContentRegistry xContentRegistry() {
     }
 
     private ModelLoadingService mockModelLoadingService() {
-        return mock(ModelLoadingService.class);
+        ModelLoadingService modelLoadingService = mock(ModelLoadingService.class);
+        when(modelLoadingService.getModelId(anyString())).thenAnswer(i -> i.getArgument(0));
+
+        return modelLoadingService;
     }
 
     @SuppressWarnings("unchecked")