First of all you should use f.select in this case. With select_tag you need to do this
<%= select_tag "mail[barrister_id]", options_from_collection_for_select(@barristers,
:id,:code), id: "barristerselect"%>
If you are using f.select then use this. Notice the empty curly braces. That is because html_options is the fourth argument for f.select helper. Checkout the documentation here.
<%= f.select :barrister_id, options_from_collection_for_select(@barristers,
:id,:code), {}, id: "barristerselect" %>