Ok I'm sure I could find the answer to this if I knew the correct terminology but anyway ...
So I have a form to create a new vendor. A vendor has_many services through services_vendors.
The new view has a couple "collection_select :service, i" and the controller does: @vendor.services_vendors << ServicesVendor.new()
which all works great unless you make an error filling out the form and then you get sent back to the form where rails handily fills out all the fields except for the services dropdowns.
Now from what I was reading I need an instance variable for the service to "remember" it's setting but I'm not sure what to set, how to set it or what to set it to.