cleanup hierarchies dependant on deleted element

lib/closure_tree/hierarchy_maintenance.rb

@@ -112,11 +112,28 @@ module ClassMethods
       # Note that the hierarchy table will be truncated.
       def rebuild!
         _ct.with_advisory_lock do
-          hierarchy_class.delete_all # not destroy_all -- we just want a simple truncate.
+          cleanup!
           roots.find_each { |n| n.send(:rebuild!) } # roots just uses the parent_id column, so this is safe.
         end
         nil
       end
+
+      def cleanup!
+        hierarchy_table = hierarchy_class.arel_table
+
+        [:descendant_id, :ancestor_id].each do |foreign_key|
+          alias_name = foreign_key.to_s.split('_').first + "s"
+          alias_table = Arel::Table.new(table_name).alias(alias_name)
+          arel_join = hierarchy_table.join(alias_table, Arel::Nodes::OuterJoin)
+                                     .on(alias_table[primary_key].eq(hierarchy_table[foreign_key]))
+                                     .join_sources
+
+          lonely_childs = hierarchy_class.joins(arel_join).where(alias_table[primary_key].eq(nil))
+          ids = lonely_childs.pluck(foreign_key)
+
+          hierarchy_class.where(hierarchy_table[foreign_key].in(ids)).delete_all
+        end
+      end
     end
   end
 end

spec/db/schema.rb

@@ -143,9 +143,6 @@
     t.integer "generations", :null => false
   end
 
-  add_foreign_key(:metal_hierarchies, :metal, :column => 'ancestor_id')
-  add_foreign_key(:metal_hierarchies, :metal, :column => 'descendant_id')
-
   create_table 'menu_items' do |t|
     t.string 'name'
     t.integer 'parent_id'

spec/hierarchy_maintenance_spec.rb

@@ -13,4 +13,43 @@
       expect(MetalHierarchy.count).to eq(hierarchy_count)
     end
   end
+
+  describe '.cleanup!' do
+    let!(:parent) { Metal.create(:value => "parent metal") }
+    let!(:child) { Metal.create(:value => "child metal", parent: parent) }
+
+    before do
+      MetalHierarchy.delete_all
+      Metal.rebuild!
+    end
+
+    context 'when an element is deleted' do
+      it 'should delete the child hierarchies' do
+        child.delete
+
+        Metal.cleanup!
+
+        expect(MetalHierarchy.where(descendant_id: child.id)).to be_empty
+        expect(MetalHierarchy.where(ancestor_id: child.id)).to be_empty
+      end
+
+      it 'should not delete the parent hierarchies' do
+        child.delete
+        Metal.cleanup!
+        expect(MetalHierarchy.where(ancestor_id: parent.id).size).to eq 1
+      end
+
+      it 'should not delete other hierarchies' do
+        other_parent = Metal.create(:value => "other parent metal")
+        other_child = Metal.create(:value => "other child metal", parent: other_parent)
+        Metal.rebuild!
+
+        child.delete
+        Metal.cleanup!
+
+        expect(MetalHierarchy.where(ancestor_id: other_parent.id).size).to eq 2
+        expect(MetalHierarchy.where(descendant_id: other_child.id).size).to eq 2
+      end
+    end
+  end
 end