Implements optional counter caching via and additional module

Author: benedikt

lib/mongoid/tree.rb

@@ -83,6 +83,7 @@ module Tree
 
     autoload :Ordering, 'mongoid/tree/ordering'
     autoload :Traversal, 'mongoid/tree/traversal'
+    autoload :CounterCaching, 'mongoid/tree/counter_caching'
 
     included do
       has_many :children, :class_name => self.name, :foreign_key => :parent_id, :inverse_of => :parent, :validate => false

lib/mongoid/tree/counter_caching.rb

@@ -0,0 +1,22 @@
+module Mongoid
+  module Tree
+    ##
+    # = Mongoid::Tree::CounterCaching
+    #
+    # Mongoid::Tree doesn't use a counter cache for the children by default.
+    # To enable counter caching for each node's children, include
+    # both Mongoid::Tree and Mongoid::Tree::CounterCaching into your document.
+    module CounterCaching
+      extend ActiveSupport::Concern
+
+      included do
+        field :children_count, :type => Integer, :default => 0
+
+        metadata = relations['parent']
+        metadata.options[:counter_cache] = true
+
+        add_counter_cache_callbacks(metadata)
+      end
+    end
+  end
+end

spec/mongoid/tree/counter_caching_spec.rb

@@ -0,0 +1,39 @@
+require 'spec_helper'
+
+describe Mongoid::Tree::CounterCaching do
+
+  subject { CounterCachedNode }
+
+  before do
+    setup_tree <<-ENDTREE
+      node1:
+        - node2:
+          - node3
+        - node4:
+          - node5
+          - node6
+        - node7
+    ENDTREE
+  end
+
+  context 'when a child gets created' do
+    it 'should calculate the counter cache' do
+      expect(node(:node1).children_count).to eq(3)
+    end
+  end
+
+  context 'when a child gets destroyed' do
+    it 'should update the counter cache' do
+      node(:node4).destroy
+      expect(node(:node1).children_count).to eq(2)
+    end
+  end
+
+  context 'when a child gets moved' do
+    it 'should update the counter cache' do
+      node(:node6).update(parent: node(:node1))
+      expect(node(:node4).children_count).to eq(1)
+      expect(node(:node1).children_count).to eq(4)
+    end
+  end
+end

spec/support/models/node.rb

@@ -29,4 +29,8 @@ class NodeWithEmbeddedDocument < Node
 
 class EmbeddedDocument
   include Mongoid::Document
-end
+end
+
+class CounterCachedNode < Node
+  include Mongoid::Tree::CounterCaching
+end