Implementing `#method_missing` is an easy way to build funky forms with ActiveModel while keeping the magic.

form.html.erb

<%= simple_form_for(@permissions) do |form| %>
  <% @tags.each do |tag| %>
    <h3><%= raw t('admin.permissions.tags', tag: tag.name) %></h3>
    <%= form.input "tags[#{tag.key}]",
          as: :check_boxes,
          collection: tag.edit_select_opts,
          label_method: :name,
          value_method: :key,
          label: false %>
  <% end %>
<% end %>

model.rb

class Permissions
  include Mongoid::Document
  
  field :tags,          type: Hash, default: {}
  
  # Defines `tags[__key__]=` and `tags[__key__]` for ActiveModel
  # form and assign magic
  def method_missing(method, *args, &block)
    tag_key = extract_tag_from_method(method)
    return super unless tags_with_access.include?(tag_key)

    if method.to_s.match(/=$/)
      tags[tag_key] = args.first
    else
      tags[tag_key]
    end
  end

  def respond_to?(method, include_private=false)
    tag_key = extract_tag_from_method(method)
    if tag_key && tags_with_access.include?(tag_key)
      true
    else
      super
    end
  end


  private

  def extract_tag_from_method(method)
    match = method.to_s.match(/^tags\[(\w+)\]=?$/)
    match ? match[1] : nil
  end

end