The workaround is to delete from the collection and then re-assign to the association:

for example, instead of:

group.users.delete(bob)

you have to do something like:

g_users = group.users.to_a

g_users.delete(bob)

group.users = g_users

group.save!

bob.destroy

I don't think this works (at least for me). As soon as I do the assignment (group.users = g_users) the sql is instantly executed to update the has_many collection. Therefore the update has been committed before I can validate the new collection.

Thanks

This seems to work for me. It is based on Railscast #75 Routing - Receptors with a dot in the name http://railscasts.com/episodes/75, where Projects have many Tasks. I also give each task a position to allow for reordering (not shown).

project.rb

has_many :tasks, :dependent => :destroy

#validation statements here...

after_update :save_tasks

def tasks=(tasks_attributes)

tasks_attributes[:new_tasks] ||= []

position = 0

#For existing tasks

tasks.each do |task|

attributes = tasks_attributes[task.id_s]

unless attributes.nil?

task.attributes = attributes

unless task.should_delete

task.position = position

position += 1

end

end

end

#For new tasks

tasks_attributes[:new_tasks].each do |task_attributes|

task_attributes[:position] = position

tasks.build(task_attributes)

position += 1

end

end

def save_tasks

tasks.each do |task|

if (task.should_delete != true)

task.save(false)

else

tasks.delete(task)

end

end

end

def id_s(element = nil)

id = self.id || self.object_id

if element.nil?

"#{self.class.to_s.underscore}_#{id}"

else

"#{self.class.to_s.underscore}_#{id}_#{element}"

end

end

protected

def validate

tasks.each_with_index do |task, position|

errors.add_to_base("Task #{position + 1} Name cannot be blank.") if (task.name.blank?)

end

end

------------------

task.rb

belongs_to :project

validates_presence_of :name

attr_accessor :should_delete

def should_delete

@should_delete || false

end

def should_delete=(bool)

if (bool == false)

@should_delete = false

else

@should_delete = true

end

end

def id_s(element = nil)

id = self.id || self.object_id

if element.nil?

"#{self.class.to_s.underscore}_#{id}"

else

"#{self.class.to_s.underscore}_#{id}_#{element}"

end

end

------------------

new.rhtml

<% form_tag({:controller => Project, :action => "new"}) do %>

New Project

<%= error_messages_for :project %>

<% end %>

------------------

_task.rhtml

This seems to work for me. It is based on Railscast 75 http://railscasts.com/episodes/75, where Projects have many Tasks. I also give each task a position to allow for reordering (not shown).

project.rb

has_many :tasks, :dependent => :destroy

#validation statements here...

after_update :save_tasks

def tasks=(tasks_attributes)
	tasks_attributes[:new_tasks] ||= []
	position = 0
	
	#For existing tasks
	tasks.each do |task|
		attributes = tasks_attributes[task.id_s]
		unless attributes.nil?
			task.attributes = attributes
			unless task.should_delete
				task.position = position
				position += 1
			end
		end
	end
	
	#For new tasks
	tasks_attributes[:new_tasks].each do |task_attributes|
		task_attributes[:position] = position
		tasks.build(task_attributes)
		position += 1
	end
end

def save_tasks
 	tasks.each do |task|
		if (task.should_delete != true)
    			task.save(false)
		else
			tasks.delete(task)
		end
	end
end

def id_s(element = nil)
	id = self.id || self.object_id
	if element.nil?
  		"#{self.class.to_s.underscore}_#{id}"
    	else
      		"#{self.class.to_s.underscore}_#{id}_#{element}"
    	end
end

protected
	
def validate
	tasks.each_with_index do |task, position|
		errors.add_to_base("Task #{position + 1} Name cannot be blank.") if (task.name.blank?)
	end
end

------------------

task.rb

belongs_to :project

validates_presence_of :name

attr_accessor :should_delete
	
def should_delete
	@should_delete || false
end
	
def should_delete=(bool)
	if (bool == false)
		@should_delete = false
	else
		@should_delete = true
	end
end
	
def id_s(element = nil)
	id = self.id || self.object_id
	if element.nil?
  		"#{self.class.to_s.underscore}_#{id}"
    	else
      		"#{self.class.to_s.underscore}_#{id}_#{element}"
    	end
end

------------------

new.rhtml

<% form_tag({:controller => Project, :action => "new"}) do %>
	<h1>New Project</h1>
	
	<%= error_messages_for :project %>

	<div class="form_div">
	  <%= label :project, :name, "Name:" %> <%= text_field :project, :name %>
	</div>
	
	<div class="form_div">
		<div id="tasks">
		 	<%= render :partial => 'task', :collection => @project.tasks %>
		</div>
		<%= link_to_remote("Add Task", :update => :tasks, :position => :bottom, :url => {:controller => Project, :action => "add_task"}) %>
	</div>

  <div class="submit_div">
		<%= submit_tag "Create", :name => 'submit_action' %>
		<%= submit_tag "Cancel", :name => 'submit_action' %>
	</div>
<% end %>

------------------

_task.rhtml

<div id="<%= task.id_s %>" class="task">
	<% unless task.new_record? %>
		<%= label_tag task.id_s("name"), "Task:" %>
		<%= text_field_tag "project[tasks][#{task.id_s}][name]", task.name, :id => task.id_s("name") %>
		<%= check_box_tag "project[tasks][#{task.id_s}][should_delete]", 1, task.should_delete, :id => task.id_s("should_delete") %>
		<%= label_tag task.id_s("should_delete"), "Delete" %>
	<% else %>
		<%= label_tag task.id_s("name"), "Task:" %>
		<%= text_field_tag "project[tasks][new_tasks][][name]", task.name, :id => task.id_s("name") %>
		<%= link_to_function "remove", "removeTask('#{task.id_s}')" %>
	<% end %>
</div>

Here is a better solution to allow for sorting of tasks using scriptaculous drag and drop.

project_controller.rb

class ProjectController < ApplicationController
	def index
		redirect_to :action => 'list'
	end
	
	def new
		submit_action = params['submit_action']
		
		if (submit_action == nil)
		  @project = Project.new
		  @project.tasks.build
		elsif (submit_action == "Create")
			@project = Project.new(params[:project])
		  if @project.save
		    flash[:notice] = "Successfully created project and tasks."
		    redirect_to :action => 'list'
		  end
		elsif (submit_action == "Cancel")
			redirect_to :action => 'list'
		end
	end

	def edit
		submit_action = params['submit_action']
		
		begin
	  	@project = Project.find(params[:id], :include => :tasks)
		rescue ActiveRecord::RecordNotFound
			flash[:notice] = "Invalid Project."
			redirect_to :action => 'list'
		else
			if (submit_action == nil)
			elsif (submit_action == "Save")
			  if @project.update_attributes(params[:project])
			    flash[:notice] = "Successfully updated project and tasks."
			    redirect_to :action => 'list'
			  end
			elsif (submit_action == "Cancel")
				redirect_to :action => 'list'
			end
		end
	end
	
	def delete
		submit_action = params['submit_action']
		
		begin
	  	@project = Project.find(params[:id])
		rescue ActiveRecord::RecordNotFound
			flash[:notice] = "Invalid Project."
			redirect_to :action => 'list'
		else
			if (submit_action == nil)
			elsif (submit_action == "Delete")
				@project.destroy
			  flash[:notice] = "Successfully deleted project and tasks."
		    redirect_to :action => 'list'
			elsif (submit_action == "Cancel")
				redirect_to :action => 'list'
			end
		end
	end
		
	def list
		@projects = Project.find(:all, :include => :tasks)
	end
	
	def add_task
		render :partial => "task", :object => Task.new()
	end
end

application_helper.rb

# Methods added to this helper will be available to all templates in the application.
module ApplicationHelper
	def label_tag(for_tag, name)
		"<label for='#{for_tag}'>#{name}</label>"
	end
end

project.rb

class Project < ActiveRecord::Base
  has_many :tasks, :dependent => :destroy

  validates_presence_of :name
	validates_uniqueness_of :name

  after_update :save_tasks

	def tasks=(tasks_attributes)
		existing_tasks = {}
		new_tasks = []
		
		tasks_attributes.each_with_index do |task_attributes, position|
			task_attributes[:position] = position
			if (task_attributes.has_key?(:id))
				id = task_attributes.delete(:id)
				existing_tasks[id] = task_attributes
			else
				new_tasks.push(task_attributes)
			end
		end
		
		#For existing tasks
		tasks.each do |task|
			attributes = existing_tasks[task.id.to_s]
			unless attributes.nil?
				task.attributes = attributes
			end
		end
		
		#For new tasks
		new_tasks.each do |task_attributes|
			tasks.build(task_attributes)
		end
	end
	  
  def save_tasks
    tasks.each do |task|
			logger.info("#{task.id}: #{task.should_delete}")
			if (task.should_delete != true)
      	task.save(false)
			else
				tasks.delete(task)
			end
    end
  end
	
	def id_s(element = nil)
		id = self.id || self.object_id
    if element.nil?
  	  "#{self.class.to_s.underscore}_#{id}"
    else
      "#{self.class.to_s.underscore}_#{id}_#{element}"
    end
  end

	protected
	
	def validate
		tasks.each_with_index do |task, position|
			errors.add_to_base("Task #{position + 1} Name cannot be blank.") if (task.name.blank?)
		end
	end
end

task.rb

class Task < ActiveRecord::Base
  belongs_to :project

  validates_presence_of :name

	attr_accessor :should_delete
	
	def <=> other_task
		position <=> other_task.position
	end
	
	def should_delete
		@should_delete || false
	end
	
	def should_delete=(bool)
		if (bool == false)
			@should_delete = false
		else
			@should_delete = true
		end
	end
	
	def id_s(element = nil)
		id = self.id || self.object_id
    if element.nil?
  	  "#{self.class.to_s.underscore}_#{id}"
    else
      "#{self.class.to_s.underscore}_#{id}_#{element}"
    end
  end
end

new.rhtml

<% form_tag({:controller => Project, :action => "new"}) do %>
	<h1>New Project</h1>
	
	<%= error_messages_for :project %>

	<div class="form_div">
	  <%= label :project, :name, "Name:" %> <%= text_field :project, :name %>
	</div>
	
	<div class="form_div">
		<div id="tasks">
		 	<%= render :partial => 'task', :collection => @project.tasks.sort %>
		</div>
		<%= javascript_tag("createTaskSortable()") %>
		<%= link_to_remote("Add Task", :update => :tasks, :position => :bottom, :url => {:controller => Project, :action => "add_task"}, :complete => "createTaskSortable()") %>
	</div>

  <div class="submit_div">
		<%= submit_tag "Create", :name => 'submit_action' %>
		<%= submit_tag "Cancel", :name => 'submit_action' %>
	</div>
<% end %>

edit.rhtml

<% form_tag({:controller => Project, :action => "edit", :id => @project.id}) do %>
	<h1>Edit Project</h1>
	
	<%= error_messages_for :project %>

	<div class="form_div">
	  <%= label :project, :name, "Name:" %> <%= text_field :project, :name %>
	</div>
	
	<div class="form_div">
		<div id="tasks">
		 	<%= render :partial => 'task', :collection => @project.tasks.sort %>
		</div>
		<%= javascript_tag("createTaskSortable()") %>
		<%= link_to_remote("Add Task", :update => :tasks, :position => :bottom, :url => {:controller => Project, :action => "add_task"}, :complete => "createTaskSortable()") %>
	</div>

  <div class="submit_div">
		<%= submit_tag "Save", :name => 'submit_action' %>
		<%= submit_tag "Cancel", :name => 'submit_action' %>
	</div>
<% end %>

_task.rhtml

<div id="<%= task.id_s %>" class="task">
	<%= image_tag("drag.png", :class => "drag_handle", :id => task.id_s("drag_handle"), :style => "cursor: move;") %>
	<% unless task.new_record? %>
		<%= hidden_field_tag "project[tasks][][id]", task.id, :id => task.id_s("id") %>
	
		<%= label_tag task.id_s("name"), "Task:" %>	<%= text_field_tag "project[tasks][][name]", task.name, :id => task.id_s("name") %>
		
		<%= check_box_tag "project[tasks][][should_delete]", 1, task.should_delete, :id => task.id_s("should_delete") %> <%= label_tag task.id_s("should_delete"), "Delete" %>
	<% else %>
		<%= label_tag task.id_s("name"), "Task:" %> <%= text_field_tag "project[tasks][][name]", task.name, :id => task.id_s("name") %>
		
		<%= link_to_function "remove", "removeTask('#{task.id_s}')" %>
	<% end %>
</div>

Hi Josh,

Just tested this again with Rails 2.3... Same code I attached with my original ticket. I still think this is a problem.