Custom filters/search in sidebar for active_admin using a partial

_search.html.haml

= form_for @search, url: collection_path,
          as: :q,
          builder: ActiveAdmin::Filters::CustomFormBuilder,
          html: {method: :get, class: :filter_form} do |f|

  = f.input :code_contains, as: :string
  = f.input :code_contains, as: :filter_string

  = f.form_buffers.last

  = f.buttons do
    = f.submit I18n.t('active_admin.filter')
    = link_to(I18n.t('active_admin.clear_filters'), "#", :class => "clear_filters_btn")
    = hidden_field_tags_for(params, :except => [:q, :page])

custom_form_builder.rb

# lib/active_admin/filters/custom_form_builder.rb
module ActiveAdmin
  module Filters
    class CustomFormBuilder < ::ActiveAdmin::Filters::FormBuilder
      def input(method, options = {})
        options = options.dup # Allow options to be shared without being tainted by Formtastic
        options[:as] ||= default_input_type(method, options)

        klass = input_class(options[:as])

        obj = klass.new(self, template, @object, @object_name, method, options)

        class << obj
          include ActiveAdmin::Inputs::FilterBase
        end

        obj.to_html
      end

      def custom_input_class_name(as)
        "#{as.to_s.camelize}Input"
      end

      def active_admin_input_class_name(as)
        "ActiveAdmin::Inputs::#{custom_input_class_name(as)}"
      end
    end
  end
end

models.rb

  config.clear_sidebar_sections!

  sidebar :filters do
    render partial: 'search'
  end

@jsyoun0823 nice that you find it helpful!