app/views/meetups/_holding_fields.html.erb
<div class="nested-fields card">
<div class="field card-body">
<%= f.label :user_id %>
<div class="form-group row">
<div class="col-sm-11">
<%= f.select(:user_id, User.all.where.not(id: current_user.id).reject { |u| %>
<% @meetup.hosts.include? u %>
<% }.collect {|u| [u.username, u.id]}, {}, {class:'form-control'}) %>
</div>
<%= f.hidden_field :meetup_id, value: @meetup.id %>
<div class="col-sm-1">
<%= link_to_remove_association f, {class:'btn btn-danger', aria: {label:"#{I18n.t 'main.remove'}"}} do %><%= icon('fas', 'times') %><% end %>
</div>
</div>
<%= f.select(:role, [["Admin", 0],["Mod", 1], ["Plain", 2]], {}, {class:'form-control'}) %>
</div>
</div>