fix: only generate links when fields expand to href [DHIS2-21856] (2.42)

Single-object GET treated any field preset (e.g. :owner) as if href was
requested, triggering deep link generation that reflectively initializes
every lazy collection of the entity. On objects with very large
collections (e.g. a user role with 240k members) this loads and
L2-caches every member entity, taking 30+ seconds of CPU per request.

Only *, :all and :simple actually expand to the transient href property,
so only those (and a literal href field) now trigger link generation.
Response payloads are unchanged for all field values.

AI Assisted

Author: netroms

dhis-2/dhis-test-web-api/src/test/java/org/hisp/dhis/webapi/controller/AbstractCrudControllerTest.java

@@ -75,6 +75,8 @@
 import org.hisp.dhis.user.UserGroup;
 import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 import org.springframework.http.MediaType;
 import org.springframework.transaction.annotation.Transactional;
 
@@ -105,6 +107,28 @@ void testGetObject() {
     assertEquals(id, userById.getId());
   }
 
+  @ParameterizedTest
+  @ValueSource(strings = {":all", "*", ":simple", "href", "id,href", "id,name,href"})
+  @DisplayName("Single object GET returns href when requested directly or via an expanding preset")
+  void testGetObjectWithHref(String fields) {
+    String id = GET("/users/").content().getList("users", JsonUser.class).get(0).getId();
+    JsonUser user =
+        GET("/users/{id}?fields={fields}", id, fields).content(HttpStatus.OK).as(JsonUser.class);
+    String href = user.getString("href").string();
+    assertNotNull(href);
+    assertTrue(href.endsWith("/users/" + id));
+  }
+
+  @ParameterizedTest
+  @ValueSource(strings = {":owner", ":identifiable", ":nameable", ":persisted", "id,name"})
+  @DisplayName("Single object GET omits href for presets that do not expand to it")
+  void testGetObjectWithoutHref(String fields) {
+    String id = GET("/users/").content().getList("users", JsonUser.class).get(0).getId();
+    JsonUser user =
+        GET("/users/{id}?fields={fields}", id, fields).content(HttpStatus.OK).as(JsonUser.class);
+    assertFalse(user.has("href"));
+  }
+
   @Test
   void testGetObjectProperty() {
     // response will look like: { "surname": <name> }

dhis-2/dhis-web-api/src/main/java/org/hisp/dhis/webapi/controller/AbstractFullReadOnlyController.java

@@ -536,8 +536,14 @@ private void handleLinksAndAccess(List<T> entityList, List<String> fields, boole
 
   private boolean fieldsContains(String match, List<String> fields) {
     for (String field : fields) {
-      // for now assume href/access if * or preset is requested
-      if (field.contains(match) || field.equals("*") || field.startsWith(":")) {
+      // only presets that expand to the href property need link generation;
+      // href is transient (not owner/persisted) and not part of the fixed
+      // identifiable/nameable presets, but it is a simple property, so it is
+      // included by *, :all and :simple
+      if (field.contains(match)
+          || field.equals("*")
+          || field.equals(":all")
+          || field.equals(":simple")) {
         return true;
       }
     }