Add relationshipLink to ElideNavigator for JSON:API relationship endpoints

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

@@ -114,6 +114,27 @@ public <R extends ElideEntity> ElideNavigatorSelector<R> navigateRelationship(@N
     return new ElideNavigator<>(dtoClass, this);
   }
 
+  @Override
+  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 new ElideNavigatorOnRelationshipLink<>() {
+      @Override
+      public String build() {
+        return route;
+      }
+
+      @Override
+      public Class<R> getDtoClass() {
+        return entityClass;
+      }
+    };
+  }
+
   /**
    * Add a sorting rule to the navigator
    * Important: You need to give the full qualified route, there is NO referencing of parent relationships.

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

@@ -6,4 +6,17 @@ public interface ElideNavigatorOnId<T extends ElideEntity> extends ElideEndpoint
 
   <R extends ElideEntity> ElideNavigatorSelector<R> navigateRelationship(Class<R> entityClass, String name);
 
+  /**
+   * Points the navigator at the JSON:API relationship endpoint
+   * ({@code /data/{type}/{id}/relationships/{name}}), used to read or modify the relationship linkage itself
+   * (e.g. PATCH/POST/DELETE to add, replace or remove members). This differs from
+   * {@link #navigateRelationship(Class, String)}, which addresses the related resource(s).
+   *
+   * <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
+   */
+  <R extends ElideEntity> ElideNavigatorOnRelationshipLink<R> relationshipLink(Class<R> entityClass, String name);
+
 }

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

@@ -0,0 +1,13 @@
+package com.faforever.commons.api.elide;
+
+/**
+ * Terminal navigator pointing at a JSON:API relationship endpoint
+ * ({@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.
+ */
+public interface ElideNavigatorOnRelationshipLink<T extends ElideEntity> {
+  String build();
+
+  Class<T> getDtoClass();
+}

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

@@ -87,6 +87,33 @@ void testNavigateFromIdToId() {
                              .build(), is("/data/mapPool/5/mapVersion/1234?include=author"));
   }
 
+  @Test
+  void testRelationshipLink() {
+    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
+  void testRelationshipLinkAfterNavigateRelationship() {
+    assertThat(ElideNavigator.of(MapPool.class)
+                             .id("5")
+                             .navigateRelationship(MapVersion.class, "mapVersion")
+                             .id("1234")
+                             .relationshipLink(MapVersion.class, "map")
+                             .build(), is("/data/mapPool/5/mapVersion/1234/relationships/map"));
+  }
+
+  @Test
+  void testCannotRelationshipLinkAfterIncludes() {
+    assertThrows(IllegalStateException.class, () -> ElideNavigator.of(MapPool.class)
+                                                                  .id("5")
+                                                                  .addInclude("mapVersion")
+                                                                  .relationshipLink(MapVersion.class, "mapVersion"));
+  }
+
   @Test
   void testGetListPages() {
     assertThat(ElideNavigator.of(MapPoolAssignment.class)