handle merging tag categories

src/app/api/tags/categories/[id]/route.ts

@@ -1,5 +1,5 @@
 import { NextRequest, NextResponse } from 'next/server'
-import { updateCategory, deleteCategory, deleteCategoryForce, getTags } from '@/lib/tags'
+import { updateCategory, deleteCategory, deleteCategoryForce, getTags, getCategories, mergeCategories } from '@/lib/tags'
 import { requireAdmin } from '@/lib/auth'
 
 export async function PATCH(
@@ -11,9 +11,30 @@ export async function PATCH(
 
   try {
     const { id } = await params
-    const { name } = await request.json()
+    const { name, merge } = await request.json()
+
+    try {
       const category = updateCategory(id, name)
       return NextResponse.json(category)
+    } catch (err) {
+      const msg = (err as Error).message
+      if (!msg.includes('already exists')) throw err
+
+      // A category with this name already exists — find it
+      const trimmed = (name as string).trim()
+      const target = getCategories().find((c) => c.name.toLowerCase() === trimmed.toLowerCase())
+      if (!target) throw err
+
+      if (merge) {
+        mergeCategories(id, target.id)
+        return NextResponse.json(target)
+      }
+
+      return NextResponse.json(
+        { error: msg, conflict: true, targetCategoryId: target.id },
+        { status: 409 }
+      )
+    }
   } catch (err) {
     return NextResponse.json({ error: (err as Error).message }, { status: 400 })
   }

src/app/manage/tags/page.tsx

@@ -85,11 +85,13 @@ function CategoryBlock({
   const [confirming, setConfirming] = useState(false)
   const [deleting, setDeleting] = useState(false)
   const [error, setError] = useState<string | null>(null)
+  const [mergeConflict, setMergeConflict] = useState<{ name: string } | null>(null)
   const cancelRef = useRef<ReturnType<typeof setTimeout> | null>(null)
 
   const handleRename = async (e: React.FormEvent) => {
     e.preventDefault()
     setError(null)
+    setMergeConflict(null)
     setSaving(true)
     try {
       const res = await fetch(`/api/tags/categories/${encodeURIComponent(category.id)}`, {
@@ -98,8 +100,35 @@ function CategoryBlock({
         body: JSON.stringify({ name: editName }),
       })
       const data = await res.json()
+      if (!res.ok) {
+        if (res.status === 409 && data.conflict) {
+          setMergeConflict({ name: editName.trim() })
+          setSaving(false)
+          return
+        }
+        setError(data.error); setSaving(false); return
+      }
+      setEditing(false)
+      onChanged()
+    } catch {
+      setError('Network error.')
+    }
+    setSaving(false)
+  }
+
+  const handleMerge = async () => {
+    setError(null)
+    setSaving(true)
+    try {
+      const res = await fetch(`/api/tags/categories/${encodeURIComponent(category.id)}`, {
+        method: 'PATCH',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ name: editName, merge: true }),
+      })
+      const data = await res.json()
       if (!res.ok) { setError(data.error); setSaving(false); return }
       setEditing(false)
+      setMergeConflict(null)
       onChanged()
     } catch {
       setError('Network error.')
@@ -158,7 +187,7 @@ function CategoryBlock({
             </button>
             <button
               type="button"
-              onClick={() => { setEditing(false); setEditName(category.name); setError(null) }}
+              onClick={() => { setEditing(false); setEditName(category.name); setError(null); setMergeConflict(null) }}
               className="text-xs px-2.5 py-1.5 rounded-lg transition-colors"
               style={{ backgroundColor: 'var(--border)', color: 'var(--text-secondary)' }}
             >
@@ -230,6 +259,32 @@ function CategoryBlock({
         </p>
       )}
 
+      {mergeConflict && (
+        <div className="mb-3 px-3 py-2 rounded-lg text-xs" style={{ backgroundColor: '#78350f33', color: '#fbbf24' }}>
+          <p className="mb-2">
+            A category named &ldquo;{mergeConflict.name}&rdquo; already exists. This will merge all tags from
+            &ldquo;{category.name}&rdquo; into it. Tags with the same name will be combined.
+          </p>
+          <div className="flex gap-2">
+            <button
+              onClick={handleMerge}
+              disabled={saving}
+              className="text-xs px-2.5 py-1.5 rounded-lg transition-colors disabled:opacity-50"
+              style={{ backgroundColor: '#b45309', color: '#fff' }}
+            >
+              {saving ? 'Merging…' : 'Merge'}
+            </button>
+            <button
+              onClick={() => setMergeConflict(null)}
+              className="text-xs px-2.5 py-1.5 rounded-lg transition-colors"
+              style={{ backgroundColor: 'var(--border)', color: 'var(--text-secondary)' }}
+            >
+              Cancel
+            </button>
+          </div>
+        </div>
+      )}
+
       {/* Tags list */}
       <div className="flex flex-wrap gap-2 mb-3">
         {tags.map((tag) => (

src/lib/tags.ts

@@ -98,6 +98,62 @@ export function deleteCategoryForce(id: string): void {
   if (result.changes === 0) throw new Error(`Category not found: ${id}`)
 }
 
+/**
+ * Merge all tags from `sourceId` category into `targetId` category, then
+ * delete the source category. Tags with conflicting names (case-insensitive)
+ * are combined: their media_tags and tag_mappings rows are re-pointed to the
+ * target tag, and the source tag is deleted.
+ */
+export function mergeCategories(sourceId: string, targetId: string): void {
+  const db = getDb()
+
+  const source = db.prepare('SELECT id, name FROM tag_categories WHERE id = ?').get(sourceId) as TagCategory | undefined
+  if (!source) throw new Error(`Source category not found: ${sourceId}`)
+  const target = db.prepare('SELECT id, name FROM tag_categories WHERE id = ?').get(targetId) as TagCategory | undefined
+  if (!target) throw new Error(`Target category not found: ${targetId}`)
+
+  const sourceTags = db
+    .prepare('SELECT id, name, category_id as categoryId FROM tags WHERE category_id = ?')
+    .all(sourceId) as Tag[]
+  const targetTags = db
+    .prepare('SELECT id, name, category_id as categoryId FROM tags WHERE category_id = ?')
+    .all(targetId) as Tag[]
+
+  const targetTagsByNameLower = new Map(targetTags.map((t) => [t.name.toLowerCase(), t]))
+
+  const txn = db.transaction(() => {
+    for (const srcTag of sourceTags) {
+      const conflict = targetTagsByNameLower.get(srcTag.name.toLowerCase())
+      if (conflict) {
+        // Re-point media_tags from source tag to target tag (ignore duplicates)
+        db.prepare(
+          `INSERT OR IGNORE INTO media_tags (item_key, tag_id)
+           SELECT item_key, ? FROM media_tags WHERE tag_id = ?`
+        ).run(conflict.id, srcTag.id)
+        db.prepare('DELETE FROM media_tags WHERE tag_id = ?').run(srcTag.id)
+
+        // Re-point tag_mappings from source tag to target tag (ignore duplicates)
+        db.prepare(
+          `UPDATE OR IGNORE tag_mappings SET tag_id = ? WHERE tag_id = ?`
+        ).run(conflict.id, srcTag.id)
+        // Delete any remaining (were duplicates that couldn't be updated)
+        db.prepare('DELETE FROM tag_mappings WHERE tag_id = ?').run(srcTag.id)
+
+        // Delete the source tag
+        db.prepare('DELETE FROM tags WHERE id = ?').run(srcTag.id)
+      } else {
+        // No conflict — just move the tag to the target category
+        db.prepare('UPDATE tags SET category_id = ? WHERE id = ?').run(targetId, srcTag.id)
+      }
+    }
+
+    // Delete the now-empty source category
+    db.prepare('DELETE FROM tag_categories WHERE id = ?').run(sourceId)
+  })
+
+  txn()
+}
+
 // ─── Tags ─────────────────────────────────────────────────────────────────────
 
 export function getTags(categoryId?: string): Tag[] {