cart.rb

class Cart < ActiveRecord::Base
  belongs_to :person    
  has_many :event_registrations, :dependent => :destroy
  has_many :membership_applications, :dependent => :destroy  
  has_one  :payment, :dependent => :destroy  
end

payment.rb

class Payment < ActiveRecord::Base
  belongs_to :cart
end

payments_controller.rb

class Admin::PaymentsController < Admin::AdminController

  def index                        
    if params[:date]   
      @show_only = DateTime.new(params[:date]['show_only(1i)'].to_i, params[:date]['show_only(2i)'].to_i)
      conditions = { :paid_at => (@show_only.beginning_of_month..@show_only.end_of_month) }     
      joins = :cart
      order = 'paid_at ASC'  
      @payments = Payment.find(:all, :conditions => conditions, :order => order, :include => joins).paginate
    else
      @payments = Payment.paginate :order => 'paid_at DESC', :page => params[:page]
    end      
  end
  
  def show
  end
  
end
              

Result of query ...

# doesn't include any joins from the :include => eager load call!
  Payment Load (0.3ms)   SELECT * FROM `payments` WHERE (`payments`.`paid_at` BETWEEN '2010-03-01 00:00:00' AND '2010-03-31 23:59:59') ORDER BY paid_at ASC