fix: clean up folder and tag assignments on tree soft delete

When a tree is soft-deleted, folder assignments and tag assignments are
now removed from junction tables. Tag usage counts are decremented with
a floor of zero to prevent negative counts.

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

backend/app/api/endpoints/trees.py

@@ -3,15 +3,15 @@ from typing import Annotated, Optional
 from uuid import UUID
 from fastapi import APIRouter, Depends, HTTPException, status, Query
 from sqlalchemy.ext.asyncio import AsyncSession
-from sqlalchemy import select, func, or_, true as sa_true
+from sqlalchemy import select, func, or_, true as sa_true, update
 from sqlalchemy.orm import selectinload
 
 from app.core.database import get_db
 from app.models.tree import Tree
 from app.models.user import User
 from app.models.category import TreeCategory
-from app.models.tag import TreeTag
+from app.models.tag import TreeTag, tree_tag_assignments
-from app.models.folder import UserFolder
+from app.models.folder import UserFolder, user_folder_trees
 from app.schemas.tree import TreeCreate, TreeUpdate, TreeResponse, TreeListResponse, CategoryInfo
 from app.api.deps import get_current_active_user, require_engineer_or_admin, require_admin
 from app.core.permissions import can_edit_tree, can_access_tree
@@ -522,6 +522,31 @@ async def delete_tree(
     tree.is_active = False
     tree.deleted_at = datetime.now(timezone.utc)
     tree.deleted_by = current_user.id
+
+    # Clean up folder assignments
+    await db.execute(
+        user_folder_trees.delete().where(user_folder_trees.c.tree_id == tree.id)
+    )
+
+    # Decrement usage_count on associated tags (floor at 0)
+    tag_ids_result = await db.execute(
+        select(tree_tag_assignments.c.tag_id).where(
+            tree_tag_assignments.c.tree_id == tree.id
+        )
+    )
+    tag_ids = [row[0] for row in tag_ids_result.fetchall()]
+    if tag_ids:
+        await db.execute(
+            update(TreeTag)
+            .where(TreeTag.id.in_(tag_ids))
+            .values(usage_count=func.greatest(TreeTag.usage_count - 1, 0))
+        )
+
+    # Clean up tag assignments
+    await db.execute(
+        tree_tag_assignments.delete().where(tree_tag_assignments.c.tree_id == tree.id)
+    )
+
     await log_audit(db, current_user.id, "tree.delete", "tree", tree.id,
                     {"tree_name": tree.name})
     await db.commit()

backend/tests/test_trees.py

@@ -2,6 +2,10 @@
 
 import pytest
 from httpx import AsyncClient
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+from app.models.folder import user_folder_trees
+from app.models.tag import tree_tag_assignments
 
 
 class TestTrees:
@@ -203,6 +207,74 @@ class TestTrees:
         tree_ids = [t["id"] for t in list_response.json()]
         assert private_tree_id in tree_ids
 
+    @pytest.mark.asyncio
+    async def test_delete_tree_cleans_up_folder_and_tag_assignments(
+        self, client: AsyncClient, auth_headers: dict, admin_auth_headers: dict, test_db: AsyncSession
+    ):
+        """Test that soft-deleting a tree removes folder and tag junction entries."""
+        # Create a tree
+        tree_data = {
+            "name": "Cascade Test Tree",
+            "description": "Will be deleted",
+            "tree_structure": {
+                "id": "root",
+                "type": "solution",
+                "title": "Test",
+                "description": "Test tree"
+            }
+        }
+        create_resp = await client.post("/api/v1/trees", json=tree_data, headers=auth_headers)
+        assert create_resp.status_code == 201
+        tree_id = create_resp.json()["id"]
+
+        # Create a folder and add the tree to it
+        folder_resp = await client.post(
+            "/api/v1/folders", json={"name": "Test Folder"}, headers=auth_headers
+        )
+        assert folder_resp.status_code == 201
+        folder_id = folder_resp.json()["id"]
+
+        add_resp = await client.post(
+            f"/api/v1/folders/{folder_id}/trees",
+            json={"tree_id": tree_id},
+            headers=auth_headers
+        )
+        assert add_resp.status_code == 201
+
+        # Add tags to the tree
+        tag_resp = await client.post(
+            f"/api/v1/tags/trees/{tree_id}",
+            json={"tags": ["cascade-test-tag"]},
+            headers=auth_headers
+        )
+        assert tag_resp.status_code == 200
+
+        # Verify junction rows exist
+        folder_rows = await test_db.execute(
+            select(user_folder_trees).where(user_folder_trees.c.tree_id == tree_id)
+        )
+        assert len(folder_rows.fetchall()) > 0
+
+        tag_rows = await test_db.execute(
+            select(tree_tag_assignments).where(tree_tag_assignments.c.tree_id == tree_id)
+        )
+        assert len(tag_rows.fetchall()) > 0
+
+        # Delete the tree (admin only)
+        del_resp = await client.delete(f"/api/v1/trees/{tree_id}", headers=admin_auth_headers)
+        assert del_resp.status_code == 204
+
+        # Verify junction rows are gone
+        folder_rows_after = await test_db.execute(
+            select(user_folder_trees).where(user_folder_trees.c.tree_id == tree_id)
+        )
+        assert len(folder_rows_after.fetchall()) == 0
+
+        tag_rows_after = await test_db.execute(
+            select(tree_tag_assignments).where(tree_tag_assignments.c.tree_id == tree_id)
+        )
+        assert len(tag_rows_after.fetchall()) == 0
+
     @pytest.mark.asyncio
     async def test_create_tree_unauthorized(self, client: AsyncClient):
         """Test that creating a tree without auth fails."""