Add an :autosave option for belongs_to associations.

lib/mongo_mapper/plugins/associations.rb

@@ -83,7 +83,7 @@ def save_to_collection(options={})
           super if defined?(super)
           associations.each do |association_name, association|
             proxy = get_proxy(association)
-            proxy.save_to_collection(options) if proxy.proxy_respond_to?(:save_to_collection)
+            proxy.save_to_collection(options) if proxy.proxy_respond_to?(:save_to_collection) && association.options[:autosave]
           end
         end
       end

lib/mongo_mapper/plugins/associations/base.rb

@@ -6,7 +6,7 @@ class Base
         attr_reader :name, :options, :query_options
 
         # Options that should not be considered MongoDB query options/criteria
-        AssociationOptions = [:as, :class, :class_name, :dependent, :extend, :foreign_key, :in, :polymorphic]
+        AssociationOptions = [:as, :class, :class_name, :dependent, :extend, :foreign_key, :in, :polymorphic, :autosave]
 
         def initialize(name, options={}, &extension)
           @name, @options, @query_options, @original_options = name, {}, {}, options

lib/mongo_mapper/plugins/associations/belongs_to_proxy.rb

@@ -30,6 +30,10 @@ def create!(attrs={})
           instantiate_target(:create!, attrs)
         end
 
+        def save_to_collection(options={})
+          @target.save(options) if @target
+        end
+
         protected
           def find_target
             return nil if proxy_owner[association.foreign_key].nil?

test/functional/associations/test_belongs_to_proxy.rb

@@ -181,4 +181,32 @@ class ::Thing
       comment.post_id.should == post.id
     end
   end
+
+  context 'autosave' do
+    should 'not be true by default' do
+      @comment_class.associations[:post].options[:autosave].should_not be_true
+    end
+
+    should 'save parent changes when true' do
+      @comment_class.associations[:post].options[:autosave] = true
+
+      comment = @comment_class.create
+      post = comment.create_post(:title => 'Hello, world!')
+
+      comment.post.attributes = {:title => 'Hi, world.'}
+      comment.save
+
+      post.reload.title.should == 'Hi, world.'
+    end
+
+    should 'not save parent changes when false' do
+      comment = @comment_class.create
+      post = comment.create_post(:title => 'Hello, world!')
+
+      comment.post.attributes = {:title => 'Hi, world.'}
+      comment.save
+
+      post.reload.title.should == 'Hello, world!'
+    end
+  end
 end