gistfile1.rbx

## /users/edit.html.erb
<% form_for [:cp, @user], :url => { :action => 'update', :id => @user.id } do |f| %>
...
<%= collection_select :users, :show_id, Show.find(:all), :id, :name, { :prompt => true } %>

## users_controller.rb
@user.shows << Show.find(params[:users][:show_id])

## error
 ActiveRecord::AssociationTypeMismatch in Cp/usersController#update

Show(#28733190) expected, got String(#104960)