Rails Select Box reject if blank
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I currently have a nested form which has select box to assign songs.

My models are songs have many setlists and vice versa, through an allocations model. Setlists accept nested attributes for allocations. 

My form is as follows:

    <div>
      <%=nested_form_for @allocation do|builder|%>
        <%=builder.label :song_id, "Pick a song" %>
    
         <%= builder.hidden_field :setlist_id, value: @setlist.id %>
    
         <%= builder.select(:song_id, options_for_select(@selections), 
         {include_blank: true}, html:{multiple: true, size: 7}) %>
    
        <%=builder.submit "Add Song", class: "btn btn-large btn-primary" %>
      <% end %>
    </div>

I want to have it so that it rejects the allocation from being created if the blank prompt is selected. I've tried:

    :reject_if => lambda { |a| a[:song_id].blank? }

but that doesn't work as the drop down box assigns the blank field a value of zero. I was wondering if anyone knew of a way around this? Thanks.