Carry the related entity type through relationshipLink

Mirror navigateRelationship by taking the related entity class:
relationshipLink(Class<R>, name) now returns ElideNavigatorOnRelationshipLink<R>,
which exposes getDtoClass() so callers can derive the related type (e.g. to build
the linkage body) instead of passing it separately.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Brutus5000

api/src/main/java/com/faforever/commons/api/elide/ElideNavigator.java

@@ -115,13 +115,24 @@ public <R extends ElideEntity> ElideNavigatorSelector<R> navigateRelationship(@N
   }
 
   @Override
-  public ElideNavigatorOnRelationshipLink relationshipLink(@NotNull String name) {
+  public <R extends ElideEntity> ElideNavigatorOnRelationshipLink<R> relationshipLink(@NotNull Class<R> entityClass,
+                                                                                      @NotNull String name) {
     if (!includes.isEmpty()) {
       throw new IllegalStateException("Cannot navigate to a relationship link with includes on parent");
     }
     log.trace("relationship link added: {}", name);
     String route = build() + "/relationships/" + name;
-    return () -> route;
+    return new ElideNavigatorOnRelationshipLink<>() {
+      @Override
+      public String build() {
+        return route;
+      }
+
+      @Override
+      public Class<R> getDtoClass() {
+        return entityClass;
+      }
+    };
   }
 
   /**

api/src/main/java/com/faforever/commons/api/elide/ElideNavigatorOnId.java

@@ -14,7 +14,9 @@ public interface ElideNavigatorOnId<T extends ElideEntity> extends ElideEndpoint
    *
    * <p>This is a terminal operation: the returned navigator only allows {@link
    * ElideNavigatorOnRelationshipLink#build()}. Includes are not allowed on the parent.
+   *
+   * @param entityClass the type of the related resource(s) the linkage points at
    */
-  ElideNavigatorOnRelationshipLink relationshipLink(String name);
+  <R extends ElideEntity> ElideNavigatorOnRelationshipLink<R> relationshipLink(Class<R> entityClass, String name);
 
 }

api/src/main/java/com/faforever/commons/api/elide/ElideNavigatorOnRelationshipLink.java

@@ -2,10 +2,12 @@
 
 /**
  * Terminal navigator pointing at a JSON:API relationship endpoint
- * ({@code /data/{type}/{id}/relationships/{name}}). It only exposes {@link #build()} because a relationship
- * link addresses the linkage itself (read or modify via PATCH/POST/DELETE) and cannot be navigated further.
+ * ({@code /data/{type}/{id}/relationships/{name}}). It only exposes {@link #build()} and the related entity
+ * type, because a relationship link addresses the linkage itself (read or modify via PATCH/POST/DELETE) and
+ * cannot be navigated further. {@code T} is the type of the related resource(s) the linkage points at.
  */
-@FunctionalInterface
-public interface ElideNavigatorOnRelationshipLink {
+public interface ElideNavigatorOnRelationshipLink<T extends ElideEntity> {
   String build();
+
+  Class<T> getDtoClass();
 }

api/src/test/java/com/faforever/commons/api/elide/ElideNavigatorTest.java

@@ -89,10 +89,11 @@ void testNavigateFromIdToId() {
 
   @Test
   void testRelationshipLink() {
-    assertThat(ElideNavigator.of(MapPool.class)
-                             .id("5")
-                             .relationshipLink("mapVersion")
-                             .build(), is("/data/mapPool/5/relationships/mapVersion"));
+    ElideNavigatorOnRelationshipLink<MapVersion> navigator = ElideNavigator.of(MapPool.class)
+                                                                           .id("5")
+                                                                           .relationshipLink(MapVersion.class, "mapVersion");
+    assertThat(navigator.build(), is("/data/mapPool/5/relationships/mapVersion"));
+    assertThat(navigator.getDtoClass(), is(MapVersion.class));
   }
 
   @Test
@@ -101,7 +102,7 @@ void testRelationshipLinkAfterNavigateRelationship() {
                              .id("5")
                              .navigateRelationship(MapVersion.class, "mapVersion")
                              .id("1234")
-                             .relationshipLink("map")
+                             .relationshipLink(MapVersion.class, "map")
                              .build(), is("/data/mapPool/5/mapVersion/1234/relationships/map"));
   }
 
@@ -110,7 +111,7 @@ void testCannotRelationshipLinkAfterIncludes() {
     assertThrows(IllegalStateException.class, () -> ElideNavigator.of(MapPool.class)
                                                                   .id("5")
                                                                   .addInclude("mapVersion")
-                                                                  .relationshipLink("mapVersion"));
+                                                                  .relationshipLink(MapVersion.class, "mapVersion"));
   }
 
   @Test