Updating a join table by directly accessing the model (See related posts)

def update
    @project = Project.find(params[:id])
    if @project.update_attributes(params[:project])
      
      for user in user_checkboxes :selected
        @project.update_assignment(user, params[:user_role][user.id.to_s])
      end
       
      for user in user_checkboxes :unselected
        @project.delete_assignment(user)
      end
      
      flash[:notice] = 'Project was successfully updated.'
      redirect_to :action => 'show', :id => @project
       
    else
      render :action => 'edit'
    end
  end

private

  def user_checkboxes(status)
    users = []
     for selected_user in params[:project_users].keys    
        choice = '1' if status == :selected
        choice = '0' if status == :unselected
        users << User.find(selected_user) if params[:project_users][selected_user] == choice
      end
    return users
  end 

class Project < ActiveRecord::Base
  has_many :assignments, :dependent => :destroy
  has_many :users, :through => :assignments
  has_many :roles, :through => :assignments

  def update_assignment(user, role_id)
    unless self.users.include?(user)
      Assignment.create(:project_id => self.id,
                        :user_id => user.id,
                        :role_id => role_id)
    else
      # update existing users
      assignment = Assignment.find_by_user_id_and_project_id(user.id, self.id)
      assignment.role_id = role_id
      assignment.save
    end
  end

  def delete_assignment(user)
    Assignment.destroy_all("user_id = #{user.id} AND project_id = #{self.id}")
  end

end