Lowest common ancestor

README.md

@@ -341,7 +341,7 @@ When you include ```has_closure_tree``` in your model, you can provide a hash to
 * ```Tag.find_or_create_by_path(path, attributes)``` returns the node whose name path is ```path```, and will create the node if it doesn't exist already.See (#find_or_create_by_path).
 * ```Tag.find_all_by_generation(generation_level)``` returns the descendant nodes who are ```generation_level``` away from a root. ```Tag.find_all_by_generation(0)``` is equivalent to ```Tag.roots```.
 * ```Tag.with_ancestor(ancestors)``` scopes to all descendants whose ancestor is in the given list.
-
+* ```Tag.lowest_common_ancestor``` finds the lowest common ancestor of the relation.
 ### Instance methods
 
 * ```tag.root``` returns the root for this node

lib/closure_tree/finders.rb

@@ -99,6 +99,19 @@ def with_descendant(*descendants)
         _ct.scope_with_order(scope)
       end
 
+      def lowest_common_ancestor
+        ancestor_id = hierarchy_class
+          .select(:ancestor_id)
+          .where(descendant_id: all)
+          .group(:ancestor_id)
+          .having("COUNT(ancestor_id) = #{count}")
+          .order(Arel.sql('MIN(generations) ASC'))
+          .limit(1)
+          .pluck(:ancestor_id).first
+
+        default_scoped(unscoped).find_by(primary_key => ancestor_id) if ancestor_id
+      end
+
       def find_all_by_generation(generation_level)
         s = joins(<<-SQL.strip_heredoc)
           INNER JOIN (

spec/tag_examples.rb

@@ -446,6 +446,59 @@ def assert_parent_and_children
       end
     end
 
+    context 'lowest_common_ancestor' do
+      let!(:t1) { tag_class.create!(name: 't1') }
+      let!(:t11) { tag_class.create!(name: 't11', parent: t1) }
+      let!(:t111) { tag_class.create!(name: 't111', parent: t11) }
+      let!(:t112) { tag_class.create!(name: 't112', parent: t11) }
+      let!(:t12) { tag_class.create!(name: 't12', parent: t1) }
+      let!(:t121) { tag_class.create!(name: 't121', parent: t12) }
+      let!(:t2) { tag_class.create!(name: 't2') }
+      let!(:t21) { tag_class.create!(name: 't21', parent: t2) }
+      let!(:t211) { tag_class.create!(name: 't211', parent: t21) }
+
+      it 'finds the parent for siblings' do
+        expect(tag_class.where(name: %w(t112 t111)).lowest_common_ancestor).to eq t11
+        expect(tag_class.where(name: %w(t12 t11)).lowest_common_ancestor).to eq t1
+      end
+
+      it 'finds the grandparent for cousins' do
+        expect(tag_class.where(name: %w(t112 N111 t121)).lowest_common_ancestor).to eq t1
+      end
+
+      it 'finds the parent/grandparent for aunt-uncle/niece-nephew' do
+        expect(tag_class.where(name: %w(t12 t112)).lowest_common_ancestor).to eq t1
+      end
+
+      it 'finds the self/parent for parent/child' do
+        expect(tag_class.where(name: %w(t12 t121)).lowest_common_ancestor).to eq t12
+        expect(tag_class.where(name: %w(t1 t12)).lowest_common_ancestor).to eq t1
+      end
+
+      it 'finds the self/grandparent for grandparent/grandchild' do
+        expect(tag_class.where(name: %w(t211 t2)).lowest_common_ancestor).to eq t2
+        expect(tag_class.where(name: %w(t111 t1)).lowest_common_ancestor).to eq t1
+      end
+
+      it 'finds the grandparent for a whole extended family' do
+        expect(tag_class.where(name: %w(t1 t11 t111 t112 t12 t121)).lowest_common_ancestor).to eq t1
+        expect(tag_class.where(name: %w(t2 t21 t211)).lowest_common_ancestor).to eq t2
+      end
+
+      it 'is nil for no items' do
+        expect(tag_class.none.lowest_common_ancestor).to be_nil
+      end
+
+      it 'is nil if there are no common ancestors' do
+        expect(tag_class.where(name: %w(t111 t211)).lowest_common_ancestor).to be_nil
+      end
+
+      it 'is itself for single item' do
+        expect(tag_class.where(name: 't111').lowest_common_ancestor).to eq t111
+        expect(tag_class.where(name: 't2').lowest_common_ancestor).to eq t2
+      end
+    end
+
     context 'paths' do
       context 'with grandchild' do
         before do