Fix similarity field mapping.

Closes spring-projects#2659

Author: sothawo

src/main/java/org/springframework/data/elasticsearch/core/index/MappingParameters.java

@@ -332,6 +332,8 @@ public void writeTypeAndParametersTo(ObjectNode objectNode) throws IOException {
 
 		if (!Similarity.Default.equals(similarity)) {
 			objectNode.put(FIELD_PARAM_SIMILARITY, similarity);
+			// similarity must have index explicitly set, otherwise Elasticsearch returns an error
+			objectNode.put(FIELD_PARAM_INDEX, index);
 		}
 
 		if (termVector != TermVector.none) {

src/test/java/org/springframework/data/elasticsearch/core/index/MappingBuilderIntegrationTests.java

@@ -263,6 +263,12 @@ void shouldWriteMappingWithFieldNameWithDots() {
 		indexOps.createWithMapping();
 	}
 
+	@Test // #2659
+	@DisplayName("should write correct mapping for dense vector property")
+	void shouldWriteCorrectMappingForDenseVectorProperty() {
+		operations.indexOps(SimilarityEntity.class).createWithMapping();
+	}
+
 	// region Entities
 	@Document(indexName = "#{@indexNameProvider.indexName()}")
 	static class Book {
@@ -893,5 +899,14 @@ private static class FieldNameDotsEntity {
 		@Nullable
 		@Field(name = "dotted.field", type = Text) private String dottedField;
 	}
+
+	@Document(indexName = "#{@indexNameProvider.indexName()}")
+	static class SimilarityEntity {
+		@Nullable
+		@Id private String id;
+
+		@Field(type = FieldType.Dense_Vector, dims = 42, similarity = "cosine") private double[] denseVector;
+	}
+
 	// endregion
 }