app/views/events/_vote_checkbox.html.erb
<% checkbox_name = "votes[#{timeslot.id}][#{attendee.id}]" %>
<%# TODO: This call shouldn't be here :( ViewModel? %>
<% checked = Vote.exists? timeslot_id: timeslot.id, attendee_id: attendee.id, available: true %>
<input name="<%= checkbox_name %>" type="hidden" value="0" />
<%= check_box_tag(checkbox_name, 1, checked) %>