Address Copilot code review comments

- Use Arel.star.count instead of counting specific column for SQL COUNT(*)
- Replace raw SQL 'DISTINCT' with Arel's .distinct method
- Document intentional use of cartesian product join in find_all_by_generation

These changes improve code consistency while maintaining the same functionality.
All tests pass with these modifications.

Author: seuros

lib/closure_tree/arel_helpers.rb

@@ -70,7 +70,7 @@ def build_hierarchy_delete_query(hierarchy_table, id)
       # Build the middle subquery with DISTINCT
       middle_subquery = Arel::SelectManager.new
       middle_subquery.from(inner_subquery)
-      middle_subquery.project(Arel.sql('DISTINCT descendant_id'))
+      middle_subquery.project(inner_subquery[:descendant_id]).distinct
 
       # Build the DELETE statement
       delete_manager = Arel::DeleteManager.new

lib/closure_tree/finders.rb

@@ -161,6 +161,9 @@ def find_all_by_generation(generation_level)
                                .as('descendants')
 
         # Build the joins
+        # Note: We intentionally use a cartesian product join (CROSS JOIN) here.
+        # This allows us to find all nodes at a specific generation level across all root nodes.
+        # The 1=1 condition creates this cartesian product in a database-agnostic way.
         join_roots = model_table
                      .join(roots_subquery)
                      .on(Arel.sql('1 = 1'))

lib/closure_tree/numeric_deterministic_ordering.rb

@@ -67,7 +67,7 @@ def _ct_sum_order_by(node = nil)
 
         query = hierarchy_table
                 .project(
-                  hierarchy_table[:ancestor_id].count.as('total_descendants'),
+                  Arel.star.count.as('total_descendants'),
                   hierarchy_table[:generations].maximum.as('max_depth')
                 )