Allow disabling TypeHints.

Original Pull Request #1788
Closes #1788

Author: sothawo

Diff for: src/main/asciidoc/reference/elasticsearch-object-mapping.adoc

@@ -34,8 +34,6 @@ The following annotations are available:
 The most important attributes are:
 ** `indexName`: the name of the index to store this entity in.
 This can contain a SpEL template expression like `"log-#{T(java.time.LocalDate).now().toString()}"`
-** `type`: [line-through]#the mapping type.
-If not set, the lowercased simple name of the class is used.# (deprecated since version 4.0)
 ** `createIndex`: flag whether to create an index on repository bootstrapping.
 Default value is _true_.
 See <<elasticsearch.repositories.autocreation>>
@@ -170,6 +168,22 @@ public class Person {
 
 NOTE: Type hints will not be written for nested Objects unless the properties type is `Object`, an interface or the actual value type does not match the properties declaration.
 
+===== Disabling Type Hints
+
+It may be necessary to disable writing of type hints when the index that should be used already exists without having the type hints defined in its mapping and with the mapping mode set to strict. In this case, writing the type hint will produce an error, as the field cannot be added automatically.
+
+Type hints can be disabled for the whole application by overriding the method `writeTypeHints()` in a configuration class derived from `AbstractElasticsearchConfiguration` (see <<elasticsearch.clients>>).
+
+As an alternativ they can be disabled for a single index with the `@Document` annotation:
+====
+[source,java]
+----
+@Document(indexName = "index", writeTypeHint = WriteTypeHint.FALSE)
+----
+====
+
+WARNING: We strongly advise against disabling Type Hints. Only do this if you are forced to. Disabling type hints can lead to documents not being retrieved correctly from Elasticsearch in case of polymorphic data or document retrieval may fail completely.
+
 ==== Geospatial Types
 
 Geospatial types like `Point` & `GeoPoint` are converted into _lat/lon_ pairs.

Diff for: src/main/java/org/springframework/data/elasticsearch/annotations/Document.java

@@ -105,4 +105,11 @@
 	 * Configuration of version management.
 	 */
 	VersionType versionType() default VersionType.EXTERNAL;
+
+	/**
+	 * Defines if type hints should be written. {@see WriteTypeHint}.
+	 *
+	 * @since 4.3
+	 */
+	WriteTypeHint writeTypeHint() default WriteTypeHint.DEFAULT;
 }

Diff for: src/main/java/org/springframework/data/elasticsearch/annotations/WriteTypeHint.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2021 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.elasticsearch.annotations;
+
+import org.springframework.data.mapping.context.MappingContext;
+
+/**
+ * Defines if type hints should be written. Used by {@link Document} annotation.
+ *
+ * @author Peter-Josef Meisch
+ * @since 4.3
+ */
+public enum WriteTypeHint {
+
+	/**
+	 * Use the global settings from the {@link MappingContext}.
+	 */
+	DEFAULT,
+	/**
+	 * Always write type hints for the entity.
+	 */
+	TRUE,
+	/**
+	 * Never write type hints for the entity.
+	 */
+	FALSE
+}

Diff for: src/main/java/org/springframework/data/elasticsearch/config/ElasticsearchConfigurationSupport.java

@@ -26,7 +26,6 @@
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.convert.converter.Converter;
 import org.springframework.core.type.filter.AnnotationTypeFilter;
-import org.springframework.data.annotation.Persistent;
 import org.springframework.data.elasticsearch.annotations.Document;
 import org.springframework.data.elasticsearch.core.RefreshPolicy;
 import org.springframework.data.elasticsearch.core.convert.ElasticsearchConverter;
@@ -72,6 +71,7 @@ public SimpleElasticsearchMappingContext elasticsearchMappingContext(
 		mappingContext.setInitialEntitySet(getInitialEntitySet());
 		mappingContext.setSimpleTypeHolder(elasticsearchCustomConversions.getSimpleTypeHolder());
 		mappingContext.setFieldNamingStrategy(fieldNamingStrategy());
+		mappingContext.setWriteTypeHints(writeTypeHints());
 
 		return mappingContext;
 	}
@@ -171,4 +171,17 @@ protected RefreshPolicy refreshPolicy() {
 	protected FieldNamingStrategy fieldNamingStrategy() {
 		return PropertyNameFieldNamingStrategy.INSTANCE;
 	}
+
+	/**
+	 * Flag specifiying if type hints (_class fields) should be written in the index. It is strongly advised to keep the
+	 * default value of {@literal true}. If you need to write to an existing index that does not have a mapping defined
+	 * for these fields and that has a strict mapping set, then it might be necessary to disable type hints. But notice
+	 * that in this case reading polymorphic types may fail.
+	 *
+	 * @return flag if type hints should be written
+	 * @since 4.3
+	 */
+	protected boolean writeTypeHints() {
+		return true;
+	}
 }