_form.html.erb

<%= form_for @project do |f| %>

  <%= f.label :name %>
  <br>   
  <%= f.text_field :name %>
  <br>
  <%= f.label :description %>
  <br>
  <%= f.text_field :description%>
  <br>

  <div>
  <h3>Assign to:</h3>
	<% User.all.each do |user| %>
	  <%= check_box_tag "project[user_ids][]", user.id %>
	  <%= user.email %><br>
	<% end %>
  </div>

  <%= f.submit "Create Project" %>
<% end %>

console

ActiveModel::MassAssignmentSecurity::Error in ProjectsController#create

Can't mass-assign protected attributes: user_ids
Rails.root: /Users/telagraphic/Documents/programming/rails_projects/projecter

Application Trace | Framework Trace | Full Trace
app/controllers/projects_controller.rb:8:in `new'
app/controllers/projects_controller.rb:8:in `create'
Request

Parameters:

{"utf8"=>"✓",
 "authenticity_token"=>"/q4GDkWy68ZL80JuZsxoBAvYsF8BXshxYf+y3BK5/Qs=",
 "project"=>{"name"=>"Tim Bob",
 "description"=>"",
 "user_ids"=>["1",
 "2"]},
 "commit"=>"Create Project"}

project.rb

class Project < ActiveRecord::Base
  attr_accessible :description, :name
  
  has_many :user_projects
  has_many :users, :through => :user_projects
  
  validates :name, :presence => true
end

project_controller.rb

  def create
    @project = Project.new(params[:project])
    if @project.save
      redirect_to current_user
    else
      render 'new'
    end
  end

user.rb

class User < ActiveRecord::Base
  attr_accessible :email, :password, :password_confirmation
  has_secure_password
  validates_uniqueness_of :email
  
  has_many :user_projects
  has_many :projects, :through => :user_projects
end

user_project.rb

class UserProject < ActiveRecord::Base
  belongs_to :project
  belongs_to :user
end