parent_id attribute writer now acts as expected. issue#1 fixed

lib/mongoid_acts_as_tree.rb

@@ -29,6 +29,13 @@ def acts_as_tree(options = {})
 					field path_field, :type => Array,  :default => [], :index => true
 					field depth_field, :type => Integer, :default => 0
 
+					self.class_eval do
+						define_method "#{parent_id_field}=" do | new_parent_id |
+							new_parent = self.class.find new_parent_id
+							new_parent.children << self
+						end
+					end
+
 					after_save      :move_children
 					validate        :will_save_tree
 					before_destroy  :destroy_descendants
@@ -64,11 +71,11 @@ def will_save_tree
 
 				def fix_position
 					if parent.nil?
-						self[parent_id_field] = nil
+						self.write_attribute parent_id_field, nil
 						self[path_field] = []
 						self[depth_field] = 0
 					else
-						self[parent_id_field] = parent._id
+						self.write_attribute parent_id_field, parent._id
 						self[path_field] = parent[path_field] + [parent._id]
 						self[depth_field] = parent[depth_field] + 1
 						self.save
@@ -181,7 +188,7 @@ def <<(object)
 					if object.descendants.include? @parent
 						object.instance_variable_set :@_cyclic, true
 					else
-						object[object.parent_id_field] = @parent._id
+						object.write_attribute object.parent_id_field, @parent._id
 						object[object.path_field] = @parent[@parent.path_field] + [@parent._id]
 						object[object.depth_field] = @parent[@parent.depth_field] + 1
 						object.instance_variable_set :@_will_move, true
@@ -201,7 +208,7 @@ def delete(object_or_id)
 							object_or_id
 					end
 
-					object[object.parent_id_field] = nil
+					object.write_attribute object.parent_id_field, nil
 					object[object.path_field]      = []
 					object[object.depth_field]     = 0
 					object.save

test/test_tree.rb

@@ -53,6 +53,37 @@ class TestMongoidActsAsTree < Test::Unit::TestCase
       assert eql_arrays?(Category.roots, [@root_1, @root_2])
     end
 
+		should "assign parent_id" do
+			child  = Category.create :name => 'child'
+			parent = Category.create :name => 'parent'
+
+			child.parent_id = parent.id
+			child.save
+
+			assert_equal parent.children.first.id, child.id
+			assert_equal parent.id, child.parent_id
+			assert parent.children.include? child
+
+			assert_equal 1, child.depth
+			assert_equal [parent.id], child.path
+
+			more_deep_child = Category.create :name => 'more deep child'
+			more_deep_child.parent_id = child.id
+
+			assert_equal child.children.first.id, more_deep_child.id
+			assert_equal child.id, more_deep_child.parent_id
+			assert child.children.include? more_deep_child
+
+			assert_equal 2, more_deep_child.depth
+			assert_equal [parent.id, child.id], more_deep_child.path
+
+			assert parent.descendants.include? child
+			assert parent.descendants.include? more_deep_child
+
+			assert more_deep_child.ancestors.include? child
+			assert more_deep_child.ancestors.include? parent
+		end
+
     context "node" do
       should "have a root" do
         assert_equal @root_1.root, @root_1