docs(elasticsearch): document field name mapping (#2349)

Author: soyuka

core/elasticsearch.md

@@ -327,8 +327,7 @@ class Tweet
 }
 ```
 
-API Platform will automatically disable write operations and snake_case document fields will
-automatically be converted to camelCase object properties during serialization.
+API Platform will automatically disable write operations.
 
 Keep in mind that it is your responsibility to populate your Elasticsearch index. To do so, you can
 use [Logstash](https://www.elastic.co/products/logstash), a custom
@@ -337,6 +336,40 @@ that suits your project (such as an [ETL](https://en.wikipedia.org/wiki/Extract,
 
 You're done! The API is now ready to use.
 
+### Mapping Field Names
+
+By default, API Platform does not convert Elasticsearch document field names: a `_source` field is
+mapped to the object property of the same name (unless [`#[SerializedName]`](serialization.md) is
+used). Use camelCase field names in your mappings, as described above.
+
+If your documents use snake_case field names, configure a name converter so that they are mapped to
+camelCase properties. There are two options.
+
+To convert names for every format, set the global `name_converter` in the API Platform
+configuration. This also changes the serialization of all other formats:
+
+```yaml
+# api/config/packages/api_platform.yaml
+api_platform:
+    name_converter: "serializer.name_converter.camel_case_to_snake_case"
+```
+
+To convert names for Elasticsearch only, override the
+`api_platform.elasticsearch.name_converter.inner_fields` service and pass it the converter:
+
+```yaml
+# api/config/services.yaml
+services:
+    api_platform.elasticsearch.name_converter.inner_fields:
+        class: ApiPlatform\Elasticsearch\Serializer\NameConverter\InnerFieldsNameConverter
+        arguments:
+            - "@serializer.name_converter.camel_case_to_snake_case"
+```
+
+> [!WARNING] The same name converter builds the field names used by the Elasticsearch filters and by
+> sorting. Once a converter is set, a filter or sort on the `firstName` property targets the
+> `first_name` field of the index.
+
 ## Filtering
 
 See how to use Elasticsearch filters and how to create Elasticsearch custom filters in the