fix: error when adding child to node with no children (#447)

Co-authored-by: Charles Perier <[email protected]>

backend/editor/entries.py

@@ -613,12 +613,12 @@ async def update_node_children(self, entry, new_children_ids):
         for child_id in children_ids:
             # Create new relationships if it doesn't exist
             query = f"""
-                MATCH ()-[r:is_child_of]->(parent:{self.project_name}:ENTRY),
-                (new_child:{self.project_name}:ENTRY)
+                MATCH (parent:{self.project_name}:ENTRY), (new_child:{self.project_name}:ENTRY)
                 WHERE parent.id = $id AND new_child.id = $child_id
+                OPTIONAL MATCH ()-[r:is_child_of]->(parent)
                 WITH parent, new_child, COUNT(r) AS rel_count
                 MERGE (new_child)-[r:is_child_of]->(parent)
-                ON CREATE SET r.position = rel_count
+                ON CREATE SET r.position = CASE WHEN rel_count IS NULL THEN 1 ELSE rel_count + 1 END
             """
             _result = await get_current_transaction().run(
                 query, {"id": entry, "child_id": child_id}