Cant save a record project-user-role

I have a two models Projects and users. I have a many to many
relation. Means that all users can be in a project and a user van be
in many porjects. A user can be a Member, Admin. i created the
relation table as projects_users and added a role_id. For this i
created a view that list the users in the project in check boxes, so
if you check the box it became an admin, so i pass an array, but when
i save i sends an ActiveRecord::ReadOnlyRecord error.

def updaterole

        @project = Project.find(params[:id])
        if 1 == 1
                 @test = @project.users.find(params[:user_ids])

               for resource in @test

                 resource.role_id = 2
                 resource.save # here is the error
               end
              flash[:notice] = 'Succeed.'
              redirect_to :action => 'resources', :id => @project
        else

              render :action => 'add'
        end

     end