rails使用：through保存has_（和_belongs_to_）许多关系

Question

所以我想保存一个HABTM关系，但我的模型上有额外的字段，所以我使用has_many并通过方法。 这是我的模特：

#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

我的表格：

        <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 %>

我的控制器：

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

当我提交时，@task.save返回false，错误数组返回project_task_users验证

更新

正如Pablo89指出我将check_box_tag重命名为此 <%= check_box_tag :user_ids, user.id, @task.users.include?(user), :name => 'project_task[user_ids][]' -%> 只有当我注释掉验证时它才会保存。如何在创建project_task时验证是否选择了用户？