ActiveAdmin Estimated Counts (with schema handling)

estimated_count.rb

# based on https://medium.com/matic-insurance/how-to-make-ruby-on-rails-activeadmin-and-large-postgresql-tables-friends-again-af199b96f25

module EstimatedCount
  extend ActiveSupport::Concern

  class_methods do
    def estimated_count
      query = <<-SQL
        select reltuples as estimated_count
        from pg_class
        INNER JOIN pg_namespace n ON n.oid = pg_class.relnamespace AND nspname = '#{schema_name}'
        where relname = '#{table_name_without_schema}'
      SQL
      result = connection.execute(query).first['estimated_count']
      format('%<number>f', number: result).to_i
    end

    private

    def table_name_without_schema
      table_name.split('.').last
    end

    def schema_name
      if table_name.include?('.')
        table_name.split('.').first
      else
        'public'
      end
    end
  end
end

render_helper.rb

module ActiveAdmin
  module RenderHelper
    def self.sidebar_with_count(dsl)
      dsl.sidebar :count, only: :index do
        filtered = params[:q].present?
        content = filtered ? collection.total_count : "#{collection.estimated_count} (estimated)"
        span(content)
      end
    end
  end
end