rails saving has_(and_belongs_to_)many relationship using :through - ruby-on-rails

Question

So i want to save a HABTM relationship but i have extra field on my model so i'm usign has_many and through method.
Here's my models:
#project_task.rb
class ProjectTask < ActiveRecord::Base
attr_accessible :description, :name, :user_id, :project_id, :user_ids
belongs_to :project
belongs_to :user #Created by
has_many :project_task_users #Here is the HABTM
has_many :users, :through => :project_task_users #AND HERE
validates :name, :presence => true
validates :project_task_users, :length => { :minimum => 1} #must have atleast 1 record in the HABTM relation
accepts_nested_attributes_for :project_task_users
end
#project_task_user.rb
class ProjectTaskUser < ActiveRecord::Base
belongs_to :user
belongs_to :project_task
end
My form:
<p>Users:</p>
<% for user in #users %>
<div>
<%= check_box_tag "project_task[user_ids][]", user.id, #task.users.include?(user) %>
<%= user.name %> - <%= user.company.name %>
</div>
<% end %>
My controller:
GET
def new_task
#project = Project.find(params[:project_id])
#task = ProjectTask.new(:project_id => #project.id)
#users = #project.users
end
POST
def new_task_post
#project = Project.find(params[:project_id])
#task = ProjectTask.new params[:project_task]
#task.user_id = current_user.id
if #task.save
redirect_to #project
else
#users = #project.users
render action:"new_task"
end
end
When i submit, the #task.save returns false and the errors array with the project_task_users validation
Update
as Pablo89 pointed out i renamed my check_box_tag to this
<%= check_box_tag :user_ids, user.id, #task.users.include?(user), :name => 'project_task[user_ids][]' -%>
and it saves only if i comment out the validation. How should i validate that a user is selected while creating a project_task?