billing_controller.rb

class BillingController < ApplicationController
  def index
    @companies = Company.includes(:transactions).merge(Transaction.pending)
  end
end

index.html.erb

    <% @companies.each do |c| %>
      <p><%= c.fantasy_name %> has <%= pluralize(c.transactions.pending.count, 'transaction')%> pending.</p>
    <% end %>

transaction.rb

class Transaction < ActiveRecord::Base

  scope :pending, -> { where(payment_status: 'pending') }