feat(#248): enrich TagTreeNodeDTO with parentId and populate documentCount via single aggregate query

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/dto/TagTreeNodeDTO.java

@@ -3,4 +3,4 @@ package org.raddatz.familienarchiv.dto;
 import java.util.List;
 import java.util.UUID;
 
-public record TagTreeNodeDTO(UUID id, String name, String color, int documentCount, List<TagTreeNodeDTO> children) {}
+public record TagTreeNodeDTO(UUID id, String name, String color, int documentCount, List<TagTreeNodeDTO> children, UUID parentId) {}

backend/src/main/java/org/raddatz/familienarchiv/service/TagService.java

@@ -141,12 +141,17 @@ public class TagService {
     }
 
     /**
-     * Returns all tags assembled into a tree. Document counts are not included here
+     * Returns all tags assembled into a tree with document counts per node.
-     * (they are populated by the controller layer if needed, or set to 0).
+     * Uses a single aggregate query to avoid N+1 behaviour.
      */
     public List<TagTreeNodeDTO> getTagTree() {
         List<Tag> all = tagRepository.findAll();
-        return buildTree(all);
+        Map<UUID, Long> counts = tagRepository.findDocumentCountsPerTag().stream()
+                .collect(Collectors.toMap(
+                        row -> (UUID) row[0],
+                        row -> (Long) row[1]
+                ));
+        return buildTree(all, counts);
     }
 
     // ─── private helpers ─────────────────────────────────────────────────────
@@ -195,7 +200,7 @@ public class TagService {
         }
     }
 
-    private List<TagTreeNodeDTO> buildTree(List<Tag> tags) {
+    private List<TagTreeNodeDTO> buildTree(List<Tag> tags, Map<UUID, Long> counts) {
         Map<UUID, List<TagTreeNodeDTO>> childrenByParent = new HashMap<>();
 
         for (Tag tag : tags) {
@@ -206,14 +211,14 @@ public class TagService {
         }
 
         for (Tag tag : tags) {
+            int documentCount = counts.getOrDefault(tag.getId(), 0L).intValue();
             TagTreeNodeDTO node = new TagTreeNodeDTO(
-                    tag.getId(), tag.getName(), tag.getColor(), 0,
+                    tag.getId(), tag.getName(), tag.getColor(), documentCount,
-                    childrenByParent.getOrDefault(tag.getId(), new ArrayList<>())
+                    childrenByParent.getOrDefault(tag.getId(), new ArrayList<>()),
+                    tag.getParentId()
             );
             if (tag.getParentId() != null) {
                 childrenByParent.get(tag.getParentId()).add(node);
-            } else {
-                childrenByParent.get(tag.getId()); // ensure root is tracked
             }
         }
 
@@ -221,9 +226,11 @@ public class TagService {
         List<TagTreeNodeDTO> roots = new ArrayList<>();
         for (Tag tag : tags) {
             if (tag.getParentId() == null) {
+                int documentCount = counts.getOrDefault(tag.getId(), 0L).intValue();
                 roots.add(new TagTreeNodeDTO(
-                        tag.getId(), tag.getName(), tag.getColor(), 0,
+                        tag.getId(), tag.getName(), tag.getColor(), documentCount,
-                        childrenByParent.getOrDefault(tag.getId(), new ArrayList<>())
+                        childrenByParent.getOrDefault(tag.getId(), new ArrayList<>()),
+                        null
                 ));
             }
         }

backend/src/test/java/org/raddatz/familienarchiv/controller/TagControllerTest.java

@@ -96,8 +96,8 @@ class TagControllerTest {
     void getTagTree_returns200_withTreeStructure() throws Exception {
         UUID parentId = UUID.randomUUID();
         UUID childId = UUID.randomUUID();
-        TagTreeNodeDTO child = new TagTreeNodeDTO(childId, "Haus", null, 0, List.of());
+        TagTreeNodeDTO child = new TagTreeNodeDTO(childId, "Haus", null, 0, List.of(), parentId);
-        TagTreeNodeDTO parent = new TagTreeNodeDTO(parentId, "Immobilie", "teal", 0, List.of(child));
+        TagTreeNodeDTO parent = new TagTreeNodeDTO(parentId, "Immobilie", "teal", 0, List.of(child), null);
         when(tagService.getTagTree()).thenReturn(List.of(parent));
 
         mockMvc.perform(get("/api/tags/tree"))

backend/src/test/java/org/raddatz/familienarchiv/service/TagServiceTest.java

@@ -197,6 +197,7 @@ class TagServiceTest {
     @Test
     void getTagTree_returnsEmptyList_whenNoTags() {
         when(tagRepository.findAll()).thenReturn(List.of());
+        when(tagRepository.findDocumentCountsPerTag()).thenReturn(List.of());
 
         assertThat(tagService.getTagTree()).isEmpty();
     }
@@ -210,6 +211,7 @@ class TagServiceTest {
                 Tag.builder().id(idB).name("Beta").build()
         );
         when(tagRepository.findAll()).thenReturn(tags);
+        when(tagRepository.findDocumentCountsPerTag()).thenReturn(List.of());
 
         var tree = tagService.getTagTree();
 
@@ -224,6 +226,7 @@ class TagServiceTest {
         Tag parent = Tag.builder().id(parentId).name("Parent").build();
         Tag child = Tag.builder().id(childId).name("Child").parentId(parentId).build();
         when(tagRepository.findAll()).thenReturn(List.of(parent, child));
+        when(tagRepository.findDocumentCountsPerTag()).thenReturn(List.of());
 
         var tree = tagService.getTagTree();
 
@@ -233,6 +236,44 @@ class TagServiceTest {
         assertThat(tree.get(0).children().get(0).id()).isEqualTo(childId);
     }
 
+    // ─── getTagTree (enriched) ────────────────────────────────────────────────
+
+    @Test
+    void getTagTree_populatesParentId_onChildNode() {
+        UUID parentId = UUID.randomUUID();
+        UUID childId = UUID.randomUUID();
+        Tag parent = Tag.builder().id(parentId).name("Parent").build();
+        Tag child = Tag.builder().id(childId).name("Child").parentId(parentId).build();
+        when(tagRepository.findAll()).thenReturn(List.of(parent, child));
+        when(tagRepository.findDocumentCountsPerTag()).thenReturn(List.of());
+
+        var tree = tagService.getTagTree();
+
+        assertThat(tree.get(0).children().get(0).parentId()).isEqualTo(parentId);
+    }
+
+    @Test
+    void getTagTree_populatesDocumentCount_fromAggregateQuery() {
+        UUID tagId = UUID.randomUUID();
+        Tag tag = Tag.builder().id(tagId).name("Tag").build();
+        when(tagRepository.findAll()).thenReturn(List.of(tag));
+        when(tagRepository.findDocumentCountsPerTag()).thenReturn(List.<Object[]>of(new Object[]{tagId, 5L}));
+
+        var tree = tagService.getTagTree();
+
+        assertThat(tree.get(0).documentCount()).isEqualTo(5);
+    }
+
+    @Test
+    void getTagTree_callsFindDocumentCountsPerTag_exactlyOnce() {
+        when(tagRepository.findAll()).thenReturn(List.of());
+        when(tagRepository.findDocumentCountsPerTag()).thenReturn(List.of());
+
+        tagService.getTagTree();
+
+        verify(tagRepository, times(1)).findDocumentCountsPerTag();
+    }
+
     // ─── resolveEffectiveColors ───────────────────────────────────────────────
 
     @Test