collection_select problem

I have an "Installation" model with the following associations to the "user" model

  belongs_to :user_id, :class_name => "User", :foreign_key => "user_id"   belongs_to :assigned_id, :class_name => "User", :foreign_key => "assigned_id"

When creating a new installation, the user_id is set to the current user, but i have a collection_select that i am using to choose the user ,from the same user table, that will be assigned to the installation under assigned_id.

<label for="installation_technician">Assigned Technician:       <%= collection_select(:installation, :assigned_id, @users, :id, :name) %>

When i create it though i get the error "User expected, Got string", i assume because it is passing the id from the selected user as a string to the assigned_id field when it should be passing a user object? How can i get this collection select to populate the assigned_id field with the user object and not the string of the selected id, or what do i have to do to make this work???

Thanks! Joel