Add support for nested object forms to ActiveRecord and the helpers in ActionPack

Signed-Off-By: Michael Koziarski <michael@koziarski.com>

[#1202 state:committed]

7 files changed, 553 insertions, 43 deletions

diff --git a/activerecord/lib/active_record.rb b/activerecord/lib/active_record.rb
index e1265b7e1e..fa804f6a73 100644
--- a/activerecord/lib/active_record.rb
+++ b/activerecord/lib/active_record.rb
@@ -46,6 +46,7 @@ module ActiveRecord
   autoload :AssociationPreload, 'active_record/association_preload'
   autoload :Associations, 'active_record/associations'
   autoload :AttributeMethods, 'active_record/attribute_methods'
+  autoload :AutosaveAssociation, 'active_record/autosave_association'
   autoload :Base, 'active_record/base'
   autoload :Calculations, 'active_record/calculations'
   autoload :Callbacks, 'active_record/callbacks'
@@ -55,6 +56,7 @@ module ActiveRecord
   autoload :Migration, 'active_record/migration'
   autoload :Migrator, 'active_record/migration'
   autoload :NamedScope, 'active_record/named_scope'
+  autoload :NestedAttributes, 'active_record/nested_attributes'
   autoload :Observing, 'active_record/observer'
   autoload :QueryCache, 'active_record/query_cache'
   autoload :Reflection, 'active_record/reflection'
diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index 8b51a38f48..7a88465f8c 100755
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -88,6 +88,18 @@ module ActiveRecord
       end unless self.new_record?
     end
 
+    private
+      # Gets the specified association instance if it responds to :loaded?, nil otherwise.
+      def association_instance_get(name)
+        association = instance_variable_get("@#{name}")
+        association if association.respond_to?(:loaded?)
+      end
+
+      # Set the specified association instance.
+      def association_instance_set(name, association)
+        instance_variable_set("@#{name}", association)
+      end
+
     # Associations are a set of macro-like class methods for tying objects together through foreign keys. They express relationships like
     # "Project has one Project Manager" or "Project belongs to a Portfolio". Each macro adds a number of methods to the class which are
     # specialized according to the collection or association symbol and the options hash. It works much the same way as Ruby's own <tt>attr*</tt>
@@ -256,6 +268,10 @@ module ActiveRecord
     # You can manipulate objects and associations before they are saved to the database, but there is some special behavior you should be
     # aware of, mostly involving the saving of associated objects.
     #
+    # Unless you enable the :autosave option on a <tt>has_one</tt>, <tt>belongs_to</tt>,
+    # <tt>has_many</tt>, or <tt>has_and_belongs_to_many</tt> association,
+    # in which case the members are always saved.
+    #
     # === One-to-one associations
     #
     # * Assigning an object to a +has_one+ association automatically saves that object and the object being replaced (if there is one), in
@@ -752,6 +768,9 @@ module ActiveRecord
       #   If true, all the associated objects are readonly through the association.
       # [:validate]
       #   If false, don't validate the associated objects when saving the parent object. true by default.
+      # [:autosave]
+      #   If true, always save any loaded members and destroy members marked for destruction, when saving the parent object. Off by default.
+      #
       # Option examples:
       #   has_many :comments, :order => "posted_on"
       #   has_many :comments, :include => :author
@@ -865,6 +884,8 @@ module ActiveRecord
       #   If true, the associated object is readonly through the association.
       # [:validate]
       #   If false, don't validate the associated object when saving the parent object. +false+ by default.
+      # [:autosave]
+      #   If true, always save the associated object or destroy it if marked for destruction, when saving the parent object. Off by default.
       #
       # Option examples:
       #   has_one :credit_card, :dependent => :destroy  # destroys the associated credit card
@@ -882,13 +903,10 @@ module ActiveRecord
         else
           reflection = create_has_one_reflection(association_id, options)
 
-          ivar = "@#{reflection.name}"
-
           method_name = "has_one_after_save_for_#{reflection.name}".to_sym
           define_method(method_name) do
-            association = instance_variable_get(ivar) if instance_variable_defined?(ivar)
-
-            if !association.nil? && (new_record? || association.new_record? || association[reflection.primary_key_name] != id)
+            association = association_instance_get(reflection.name)
+            if association && (new_record? || association.new_record? || association[reflection.primary_key_name] != id)
               association[reflection.primary_key_name] = id
               association.save(true)
             end
@@ -979,6 +997,8 @@ module ActiveRecord
       #   If true, the associated object is readonly through the association.
       # [:validate]
       #   If false, don't validate the associated objects when saving the parent object. +false+ by default.
+      # [:autosave]
+      #   If true, always save the associated object or destroy it if marked for destruction, when saving the parent object. Off by default.
       #
       # Option examples:
       #   belongs_to :firm, :foreign_key => "client_of"
@@ -991,15 +1011,12 @@ module ActiveRecord
       def belongs_to(association_id, options = {})
         reflection = create_belongs_to_reflection(association_id, options)
 
-        ivar = "@#{reflection.name}"
-
         if reflection.options[:polymorphic]
           association_accessor_methods(reflection, BelongsToPolymorphicAssociation)
 
           method_name = "polymorphic_belongs_to_before_save_for_#{reflection.name}".to_sym
           define_method(method_name) do
-            association = instance_variable_get(ivar) if instance_variable_defined?(ivar)
-
+            association = association_instance_get(reflection.name)
             if association && association.target
               if association.new_record?
                 association.save(true)
@@ -1019,9 +1036,7 @@ module ActiveRecord
 
           method_name = "belongs_to_before_save_for_#{reflection.name}".to_sym
           define_method(method_name) do
-            association = instance_variable_get(ivar) if instance_variable_defined?(ivar)
-
-            if !association.nil?
+            if association = association_instance_get(reflection.name)
               if association.new_record?
                 association.save(true)
               end
@@ -1196,6 +1211,8 @@ module ActiveRecord
       #   If true, all the associated objects are readonly through the association.
       # [:validate]
       #   If false, don't validate the associated objects when saving the parent object. +true+ by default.
+      # [:autosave]
+      #   If true, always save any loaded members and destroy members marked for destruction, when saving the parent object. Off by default.
       #
       # Option examples:
       #   has_and_belongs_to_many :projects
@@ -1243,33 +1260,30 @@ module ActiveRecord
         end
 
         def association_accessor_methods(reflection, association_proxy_class)
-          ivar = "@#{reflection.name}"
-
           define_method(reflection.name) do |*params|
             force_reload = params.first unless params.empty?
-
-            association = instance_variable_get(ivar) if instance_variable_defined?(ivar)
+            association = association_instance_get(reflection.name)
 
             if association.nil? || force_reload
               association = association_proxy_class.new(self, reflection)
               retval = association.reload
               if retval.nil? and association_proxy_class == BelongsToAssociation
-                instance_variable_set(ivar, nil)
+                association_instance_set(reflection.name, nil)
                 return nil
               end
-              instance_variable_set(ivar, association)
+              association_instance_set(reflection.name, association)
             end
 
             association.target.nil? ? nil : association
           end
 
           define_method("loaded_#{reflection.name}?") do
-            association = instance_variable_get(ivar) if instance_variable_defined?(ivar)
+            association = association_instance_get(reflection.name)
             association && association.loaded?
           end
 
           define_method("#{reflection.name}=") do |new_value|
-            association = instance_variable_get(ivar) if instance_variable_defined?(ivar)
+            association = association_instance_get(reflection.name)
 
             if association.nil? || association.target != new_value
               association = association_proxy_class.new(self, reflection)
@@ -1280,7 +1294,7 @@ module ActiveRecord
               self.send(reflection.name, new_value)
             else
               association.replace(new_value)
-              instance_variable_set(ivar, new_value.nil? ? nil : association)
+              association_instance_set(reflection.name, new_value.nil? ? nil : association)
             end
           end
 
@@ -1288,20 +1302,18 @@ module ActiveRecord
             return if target.nil? and association_proxy_class == BelongsToAssociation
             association = association_proxy_class.new(self, reflection)
             association.target = target
-            instance_variable_set(ivar, association)
+            association_instance_set(reflection.name, association)
           end
         end
 
         def collection_reader_method(reflection, association_proxy_class)
           define_method(reflection.name) do |*params|
-            ivar = "@#{reflection.name}"
-
             force_reload = params.first unless params.empty?
-            association = instance_variable_get(ivar) if instance_variable_defined?(ivar)
+            association = association_instance_get(reflection.name)
 
-            unless association.respond_to?(:loaded?)
+            unless association
               association = association_proxy_class.new(self, reflection)
-              instance_variable_set(ivar, association)
+              association_instance_set(reflection.name, association)
             end
 
             association.reload if force_reload
@@ -1339,8 +1351,7 @@ module ActiveRecord
         def add_single_associated_validation_callbacks(association_name)
           method_name = "validate_associated_records_for_#{association_name}".to_sym
           define_method(method_name) do
-            association = instance_variable_get("@#{association_name}")
-            if !association.nil?
+            if association = association_instance_get(association_name)
               errors.add association_name unless association.target.nil? || association.valid?
             end
           end
@@ -1350,12 +1361,10 @@ module ActiveRecord
 
         def add_multiple_associated_validation_callbacks(association_name)
           method_name = "validate_associated_records_for_#{association_name}".to_sym
-          ivar = "@#{association_name}"
-
           define_method(method_name) do
-            association = instance_variable_get(ivar) if instance_variable_defined?(ivar)
+            association = association_instance_get(association_name)
 
-            if association.respond_to?(:loaded?)
+            if association
               if new_record?
                 association
               elsif association.loaded?
@@ -1372,8 +1381,6 @@ module ActiveRecord
         end
 
         def add_multiple_associated_save_callbacks(association_name)
-          ivar = "@#{association_name}"
-
           method_name = "before_save_associated_records_for_#{association_name}".to_sym
           define_method(method_name) do
             @new_record_before_save = new_record?
@@ -1383,13 +1390,13 @@ module ActiveRecord
 
           method_name = "after_create_or_update_associated_records_for_#{association_name}".to_sym
           define_method(method_name) do
-            association = instance_variable_get(ivar) if instance_variable_defined?(ivar)
+            association = association_instance_get(association_name)
 
             records_to_save = if @new_record_before_save
               association
-            elsif association.respond_to?(:loaded?) && association.loaded?
+            elsif association && association.loaded?
               association.select { |record| record.new_record? }
-            elsif association.respond_to?(:loaded?) && !association.loaded?
+            elsif association && !association.loaded?
               association.target.select { |record| record.new_record? }
             else
               []
@@ -1407,15 +1414,13 @@ module ActiveRecord
 
         def association_constructor_method(constructor, reflection, association_proxy_class)
           define_method("#{constructor}_#{reflection.name}") do |*params|
-            ivar = "@#{reflection.name}"
-
             attributees      = params.first unless params.empty?
             replace_existing = params[1].nil? ? true : params[1]
-            association      = instance_variable_get(ivar) if instance_variable_defined?(ivar)
+            association      = association_instance_get(reflection.name)
 
-            if association.nil?
+            unless association
               association = association_proxy_class.new(self, reflection)
-              instance_variable_set(ivar, association)
+              association_instance_set(reflection.name, association)
             end
 
             if association_proxy_class == HasOneAssociation
diff --git a/activerecord/lib/active_record/autosave_association.rb b/activerecord/lib/active_record/autosave_association.rb
new file mode 100644
index 0000000000..07660ebd03
--- /dev/null
+++ b/activerecord/lib/active_record/autosave_association.rb
@@ -0,0 +1,213 @@
+module ActiveRecord
+  # AutosaveAssociation is a module that takes care of automatically saving
+  # your associations when the parent is saved. In addition to saving, it
+  # also destroys any associations that were marked for destruction.
+  # (See mark_for_destruction and marked_for_destruction?)
+  #
+  # Saving of the parent, its associations, and the destruction of marked
+  # associations, all happen inside 1 transaction. This should never leave the
+  # database in an inconsistent state after, for instance, mass assigning
+  # attributes and saving them.
+  #
+  # If validations for any of the associations fail, their error messages will
+  # be applied to the parent.
+  #
+  # Note that it also means that associations marked for destruction won't
+  # be destroyed directly. They will however still be marked for destruction.
+  #
+  # === One-to-one Example
+  #
+  # Consider a Post model with one Author:
+  #
+  #   class Post
+  #     has_one :author, :autosave => true
+  #   end
+  #
+  # Saving changes to the parent and its associated model can now be performed
+  # automatically _and_ atomically:
+  #
+  #   post = Post.find(1)
+  #   post.title # => "The current global position of migrating ducks"
+  #   post.author.name # => "alloy"
+  #
+  #   post.title = "On the migration of ducks"
+  #   post.author.name = "Eloy Duran"
+  #
+  #   post.save
+  #   post.reload
+  #   post.title # => "On the migration of ducks"
+  #   post.author.name # => "Eloy Duran"
+  #
+  # Destroying an associated model, as part of the parent's save action, is as
+  # simple as marking it for destruction:
+  #
+  #   post.author.mark_for_destruction
+  #   post.author.marked_for_destruction? # => true
+  #
+  # Note that the model is _not_ yet removed from the database:
+  #   id = post.author.id
+  #   Author.find_by_id(id).nil? # => false
+  #
+  #   post.save
+  #   post.reload.author # => nil
+  #
+  # Now it _is_ removed from the database:
+  #   Author.find_by_id(id).nil? # => true
+  #
+  # === One-to-many Example
+  #
+  # Consider a Post model with many Comments:
+  #
+  #   class Post
+  #     has_many :comments, :autosave => true
+  #   end
+  #
+  # Saving changes to the parent and its associated model can now be performed
+  # automatically _and_ atomically:
+  #
+  #   post = Post.find(1)
+  #   post.title # => "The current global position of migrating ducks"
+  #   post.comments.first.body # => "Wow, awesome info thanks!"
+  #   post.comments.last.body # => "Actually, your article should be named differently."
+  #
+  #   post.title = "On the migration of ducks"
+  #   post.comments.last.body = "Actually, your article should be named differently. [UPDATED]: You are right, thanks."
+  #
+  #   post.save
+  #   post.reload
+  #   post.title # => "On the migration of ducks"
+  #   post.comments.last.body # => "Actually, your article should be named differently. [UPDATED]: You are right, thanks."
+  #
+  # Destroying one of the associated models members, as part of the parent's
+  # save action, is as simple as marking it for destruction:
+  #
+  #   post.comments.last.mark_for_destruction
+  #   post.comments.last.marked_for_destruction? # => true
+  #   post.comments.length # => 2
+  #
+  # Note that the model is _not_ yet removed from the database:
+  #   id = post.comments.last.id
+  #   Comment.find_by_id(id).nil? # => false
+  #
+  #   post.save
+  #   post.reload.comments.length # => 1
+  #
+  # Now it _is_ removed from the database:
+  #   Comment.find_by_id(id).nil? # => true
+  #
+  # === Validation
+  #
+  # Validation is performed on the parent as usual, but also on all autosave
+  # enabled associations. If any of the associations fail validation, its
+  # error messages will be applied on the parents errors object and validation
+  # of the parent will fail.
+  #
+  # Consider a Post model with Author which validates the presence of its name
+  # attribute:
+  #
+  #   class Post
+  #     has_one :author, :autosave => true
+  #   end
+  #
+  #   class Author
+  #     validates_presence_of :name
+  #   end
+  #
+  #   post = Post.find(1)
+  #   post.author.name = ''
+  #   post.save # => false
+  #   post.errors # => #<ActiveRecord::Errors:0x174498c @errors={"author_name"=>["can't be blank"]}, @base=#<Post ...>>
+  #
+  # No validations will be performed on the associated models when validations
+  # are skipped for the parent:
+  #
+  #   post = Post.find(1)
+  #   post.author.name = ''
+  #   post.save(false) # => true
+  module AutosaveAssociation
+    def self.included(base)
+      base.class_eval do
+        alias_method_chain :reload, :autosave_associations
+        alias_method_chain :save,   :autosave_associations
+        alias_method_chain :valid?, :autosave_associations
+
+        %w{ has_one belongs_to has_many has_and_belongs_to_many }.each do |type|
+          base.send("valid_keys_for_#{type}_association") << :autosave
+        end
+      end
+    end
+
+    # Saves the parent, <tt>self</tt>, and any loaded autosave associations.
+    # In addition, it destroys all children that were marked for destruction
+    # with mark_for_destruction.
+    #
+    # This all happens inside a transaction, _if_ the Transactions module is included into
+    # ActiveRecord::Base after the AutosaveAssociation module, which it does by default.
+    def save_with_autosave_associations(perform_validation = true)
+      returning(save_without_autosave_associations(perform_validation)) do |valid|
+        if valid
+          self.class.reflect_on_all_autosave_associations.each do |reflection|
+            if (association = association_instance_get(reflection.name)) && association.loaded?
+              if association.is_a?(Array)
+                association.proxy_target.each do |child|
+                  child.marked_for_destruction? ? child.destroy : child.save(perform_validation)
+                end
+              else
+                association.marked_for_destruction? ? association.destroy : association.save(perform_validation)
+              end
+            end
+          end
+        end
+      end
+    end
+
+    # Returns whether or not the parent, <tt>self</tt>, and any loaded autosave associations are valid.
+    def valid_with_autosave_associations?
+      if valid_without_autosave_associations?
+        self.class.reflect_on_all_autosave_associations.all? do |reflection|
+          if (association = association_instance_get(reflection.name)) && association.loaded?
+            if association.is_a?(Array)
+              association.proxy_target.all? { |child| autosave_association_valid?(reflection, child) }
+            else
+              autosave_association_valid?(reflection, association)
+            end
+          else
+            true # association not loaded yet, so it should be valid
+          end
+        end
+      else
+        false # self was not valid
+      end
+    end
+
+    # Returns whether or not the association is valid and applies any errors to the parent, <tt>self</tt>, if it wasn't.
+    def autosave_association_valid?(reflection, association)
+      returning(association.valid?) do |valid|
+        association.errors.each do |attribute, message|
+          errors.add "#{reflection.name}_#{attribute}", message
+        end unless valid
+      end
+    end
+
+    # Reloads the attributes of the object as usual and removes a mark for destruction.
+    def reload_with_autosave_associations(options = nil)
+      @marked_for_destruction = false
+      reload_without_autosave_associations(options)
+    end
+
+    # Marks this record to be destroyed as part of the parents save transaction.
+    # This does _not_ actually destroy the record yet, rather it will be destroyed when <tt>parent.save</tt> is called.
+    #
+    # Only useful if the <tt>:autosave</tt> option on the parent is enabled for this associated model.
+    def mark_for_destruction
+      @marked_for_destruction = true
+    end
+
+    # Returns whether or not this record will be destroyed as part of the parents save transaction.
+    #
+    # Only useful if the <tt>:autosave</tt> option on the parent is enabled for this associated model.
+    def marked_for_destruction?
+      @marked_for_destruction
+    end
+  end
+end
\ No newline at end of file
diff --git a/activerecord/lib/active_record/base.rb b/activerecord/lib/active_record/base.rb
index 0efccb66ee..f9168c8dc2 100755
--- a/activerecord/lib/active_record/base.rb
+++ b/activerecord/lib/active_record/base.rb
@@ -3136,6 +3136,11 @@ module ActiveRecord #:nodoc:
     include Dirty
     include Callbacks, Observing, Timestamp
     include Associations, AssociationPreload, NamedScope
+
+    # AutosaveAssociation needs to be included before Transactions, because we want
+    # #save_with_autosave_associations to be wrapped inside a transaction.
+    include AutosaveAssociation, NestedAttributes
+
     include Aggregations, Transactions, Reflection, Calculations, Serialization
   end
 end
diff --git a/activerecord/lib/active_record/nested_attributes.rb b/activerecord/lib/active_record/nested_attributes.rb
new file mode 100644
index 0000000000..8bfdadd0e3
--- /dev/null
+++ b/activerecord/lib/active_record/nested_attributes.rb
@@ -0,0 +1,279 @@
+module ActiveRecord
+  module NestedAttributes #:nodoc:
+    def self.included(base)
+      base.extend(ClassMethods)
+      base.class_inheritable_accessor :reject_new_nested_attributes_procs, :instance_writer => false
+      base.reject_new_nested_attributes_procs = {}
+    end
+
+    # == Nested Attributes
+    #
+    # Nested attributes allow you to save attributes on associated records
+    # through the parent. By default nested attribute updating is turned off,
+    # you can enable it using the accepts_nested_attributes_for class method.
+    # When you enable nested attributes an attribute writer is defined on
+    # the model.
+    #
+    # The attribute writer is named after the association, which means that
+    # in the following example, two new methods are added to your model:
+    # <tt>author_attributes=(attributes)</tt> and
+    # <tt>pages_attributes=(attributes)</tt>.
+    #
+    #   class Book < ActiveRecord::Base
+    #     has_one :author
+    #     has_many :pages
+    #
+    #     accepts_nested_attributes_for :author, :pages
+    #   end
+    #
+    # Note that the <tt>:autosave</tt> option is automatically enabled on every
+    # association that accepts_nested_attributes_for is used for.
+    #
+    # === One-to-one
+    #
+    # Consider a Member model that has one Avatar:
+    #
+    #   class Member < ActiveRecord::Base
+    #     has_one :avatar
+    #     accepts_nested_attributes_for :avatar
+    #   end
+    #
+    # Enabling nested attributes on a one-to-one association allows you to
+    # create the member and avatar in one go:
+    #
+    #   params = { 'member' => { 'name' => 'Jack', 'avatar_attributes' => { 'icon' => 'smiling' } } }
+    #   member = Member.create(params)
+    #   member.avatar.icon #=> 'smiling'
+    #
+    # It also allows you to update the avatar through the member:
+    #
+    #   params = { 'member' => { 'avatar_attributes' => { 'icon' => 'sad' } } }
+    #   member.update_attributes params['member']
+    #   member.avatar.icon #=> 'sad'
+    #
+    # By default you will only be able to set and update attributes on the
+    # associated model. If you want to destroy the associated model through the
+    # attributes hash, you have to enable it first using the
+    # <tt>:allow_destroy</tt> option.
+    #
+    #   class Member < ActiveRecord::Base
+    #     has_one :avatar
+    #     accepts_nested_attributes_for :avatar, :allow_destroy => true
+    #   end
+    #
+    # Now, when you add the <tt>_delete</tt> key to the attributes hash, with a
+    # value that evaluates to +true+, you will destroy the associated model:
+    #
+    #   member.avatar_attributes = { '_delete' => '1' }
+    #   member.avatar.marked_for_destruction? # => true
+    #   member.save
+    #   member.avatar #=> nil
+    #
+    # Note that the model will _not_ be destroyed until the parent is saved.
+    #
+    # === One-to-many
+    #
+    # Consider a member that has a number of posts:
+    #
+    #   class Member < ActiveRecord::Base
+    #     has_many :posts
+    #     accepts_nested_attributes_for :posts, :reject_if => proc { |attributes| attributes['title'].blank? }
+    #   end
+    #
+    # You can now set or update attributes on an associated post model through
+    # the attribute hash.
+    #
+    # For each key in the hash that starts with the string 'new' a new model
+    # will be instantiated. When the proc given with the <tt>:reject_if</tt>
+    # option evaluates to +false+ for a certain attribute hash no record will
+    # be built for that hash.
+    #
+    #   params = { 'member' => {
+    #     'name' => 'joe', 'posts_attributes' => {
+    #       'new_12345' => { 'title' => 'Kari, the awesome Ruby documentation browser!' },
+    #       'new_54321' => { 'title' => 'The egalitarian assumption of the modern citizen' },
+    #       'new_67890' => { 'title' => '' } # This one matches the :reject_if proc and will not be instantiated.
+    #     }
+    #   }}
+    #
+    #   member = Member.create(params['member'])
+    #   member.posts.length #=> 2
+    #   member.posts.first.title #=> 'Kari, the awesome Ruby documentation browser!'
+    #   member.posts.second.title #=> 'The egalitarian assumption of the modern citizen'
+    #
+    # When the key for post attributes is an integer, the associated post with
+    # that ID will be updated:
+    #
+    #   member.attributes = {
+    #     'name' => 'Joe',
+    #     'posts_attributes' => {
+    #       '1' => { 'title' => '[UPDATED] An, as of yet, undisclosed awesome Ruby documentation browser!' },
+    #       '2' => { 'title' => '[UPDATED] other post' }
+    #     }
+    #   }
+    #
+    # By default the associated models are protected from being destroyed. If
+    # you want to destroy any of the associated models through the attributes
+    # hash, you have to enable it first using the <tt>:allow_destroy</tt>
+    # option.
+    #
+    # This will allow you to specify which models to destroy in the attributes
+    # hash by setting the '_delete' attribute to a value that evaluates to
+    # +true+:
+    #
+    #   class Member < ActiveRecord::Base
+    #     has_many :posts
+    #     accepts_nested_attributes_for :posts, :allow_destroy => true
+    #   end
+    #
+    #   params = {'member' => { 'name' => 'joe', 'posts_attributes' => {
+    #     '2' => { '_delete' => '1' }
+    #   }}}
+    #   member.attributes = params['member']
+    #   member.posts.detect { |p| p.id == 2 }.marked_for_destruction? # => true
+    #   member.posts.length #=> 2
+    #   member.save
+    #   member.posts.length # => 1
+    #
+    # === Saving
+    #
+    # All changes to models, including the destruction of those marked for
+    # destruction, are saved and destroyed automatically and atomically when
+    # the parent model is saved. This happens inside the transaction initiated
+    # by the parents save method. See ActiveRecord::AutosaveAssociation.
+    module ClassMethods
+      # Defines an attributes writer for the specified association(s).
+      #
+      # Supported options:
+      # [:allow_destroy]
+      #   If true, destroys any members from the attributes hash with a
+      #   <tt>_delete</tt> key and a value that converts to +true+
+      #   (eg. 1, '1', true, or 'true'). This option is off by default.
+      # [:reject_if]
+      #   Allows you to specify a Proc that checks whether a record should be
+      #   built for a certain attribute hash. The hash is passed to the Proc
+      #   and the Proc should return either +true+ or +false+. When no Proc
+      #   is specified a record will be built for all attribute hashes.
+      #
+      # Examples:
+      #   accepts_nested_attributes_for :avatar
+      #   accepts_nested_attributes_for :avatar, :allow_destroy => true
+      #   accepts_nested_attributes_for :avatar, :reject_if => proc { ... }
+      #   accepts_nested_attributes_for :avatar, :posts, :allow_destroy => true, :reject_if => proc { ... }
+      def accepts_nested_attributes_for(*attr_names)
+        options = { :allow_destroy => false }
+        options.update(attr_names.extract_options!)
+        options.assert_valid_keys(:allow_destroy, :reject_if)
+
+        attr_names.each do |association_name|
+          if reflection = reflect_on_association(association_name)
+            type = case reflection.macro
+            when :has_one, :belongs_to
+              :one_to_one
+            when :has_many, :has_and_belongs_to_many
+              :collection
+            end
+
+            reflection.options[:autosave] = true
+            self.reject_new_nested_attributes_procs[association_name.to_sym] = options[:reject_if]
+
+            # def pirate_attributes=(attributes)
+            #   assign_nested_attributes_for_one_to_one_association(:pirate, attributes, false)
+            # end
+            class_eval %{
+              def #{association_name}_attributes=(attributes)
+                assign_nested_attributes_for_#{type}_association(:#{association_name}, attributes, #{options[:allow_destroy]})
+              end
+            }, __FILE__, __LINE__
+          else
+            raise ArgumentError, "No association found for name `#{association_name}'. Has it been defined yet?"
+          end
+        end
+      end
+    end
+
+    # Returns ActiveRecord::AutosaveAssociation::marked_for_destruction?
+    # It's used in conjunction with fields_for to build a form element
+    # for the destruction of this association.
+    #
+    # See ActionView::Helpers::FormHelper::fields_for for more info.
+    def _delete
+      marked_for_destruction?
+    end
+
+    private
+
+    # Assigns the given attributes to the association. An association will be
+    # build if it doesn't exist yet.
+    def assign_nested_attributes_for_one_to_one_association(association_name, attributes, allow_destroy)
+      if should_destroy_nested_attributes_record?(allow_destroy, attributes)
+        send(association_name).mark_for_destruction
+      else
+        (send(association_name) || send("build_#{association_name}")).attributes = attributes
+      end
+    end
+
+    # Assigns the given attributes to the collection association.
+    #
+    # Keys containing an ID for an associated record will update that record.
+    # Keys starting with <tt>new</tt> will instantiate a new record for that
+    # association.
+    #
+    # For example:
+    #
+    #   assign_nested_attributes_for_collection_association(:people, {
+    #     '1' => { 'name' => 'Peter' },
+    #     'new_43' => { 'name' => 'John' }
+    #   })
+    #
+    # Will update the name of the Person with ID 1 and create a new associated
+    # person with the name 'John'.
+    def assign_nested_attributes_for_collection_association(association_name, attributes, allow_destroy)
+      unless attributes.is_a?(Hash)
+        raise ArgumentError, "Hash expected, got #{attributes.class.name} (#{attributes.inspect})"
+      end
+
+      # Make sure any new records sorted by their id before they're build.
+      sorted_by_id = attributes.sort_by { |id, _| id.is_a?(String) ? id.sub(/^new_/, '').to_i : id }
+
+      sorted_by_id.each do |id, record_attributes|
+        if id.acts_like?(:string) && id.starts_with?('new_')
+          build_new_nested_attributes_record(association_name, record_attributes)
+        else
+          assign_to_or_destroy_nested_attributes_record(association_name, id, record_attributes, allow_destroy)
+        end
+      end
+    end
+
+    # Returns +true+ if <tt>allow_destroy</tt> is enabled and the attributes
+    # contains a truthy value for the key <tt>'_delete'</tt>.
+    #
+    # It will _always_ remove the <tt>'_delete'</tt> key, if present.
+    def should_destroy_nested_attributes_record?(allow_destroy, attributes)
+      ConnectionAdapters::Column.value_to_boolean(attributes.delete('_delete')) && allow_destroy
+    end
+
+    # Builds a new record with the given attributes.
+    #
+    # If a <tt>:reject_if</tt> proc exists for this association, it will be
+    # called with the attributes as its argument. If the proc returns a truthy
+    # value, the record is _not_ build.
+    def build_new_nested_attributes_record(association_name, attributes)
+      if reject_proc = self.class.reject_new_nested_attributes_procs[association_name]
+        return if reject_proc.call(attributes)
+      end
+      send(association_name).build(attributes)
+    end
+
+    # Assigns the attributes to the record specified by +id+. Or marks it for
+    # destruction if #should_destroy_nested_attributes_record? returns +true+.
+    def assign_to_or_destroy_nested_attributes_record(association_name, id, attributes, allow_destroy)
+      record = send(association_name).detect { |record| record.id == id.to_i }
+      if should_destroy_nested_attributes_record?(allow_destroy, attributes)
+        record.mark_for_destruction
+      else
+        record.attributes = attributes
+      end
+    end
+  end
+end
\ No newline at end of file
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 1937abdc83..e69bfb1355 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -65,6 +65,11 @@ module ActiveRecord
       def reflect_on_association(association)
         reflections[association].is_a?(AssociationReflection) ? reflections[association] : nil
       end
+
+      # Returns an array of AssociationReflection objects for all associations which have <tt>:autosave</tt> enabled.
+      def reflect_on_all_autosave_associations
+        reflections.values.select { |reflection| reflection.options[:autosave] }
+      end
     end
 
 
diff --git a/activerecord/lib/active_record/test_case.rb b/activerecord/lib/active_record/test_case.rb
index 149b93203e..211dd78874 100644
--- a/activerecord/lib/active_record/test_case.rb
+++ b/activerecord/lib/active_record/test_case.rb
@@ -27,6 +27,7 @@ module ActiveRecord
       $queries_executed = []
       yield
     ensure
+      %w{ BEGIN COMMIT }.each { |x| $queries_executed.delete(x) }
       assert_equal num, $queries_executed.size, "#{$queries_executed.size} instead of #{num} queries were executed.#{$queries_executed.size == 0 ? '' : "\nQueries:\n#{$queries_executed.join("\n")}"}"
     end