## all_diff will calculate some cumulative fields and save them back to the db

## Model
class Reading < ActiveRecord::Base

  def self.all_diff(unit_id)
    first = find(:first, :conditions =>  ["unit_id = ?", unit_id], :order => "date asc" )
    counter_last = first.counter
    date_last = first.date

    find(:all, :conditions =>  ["unit_id = ?", unit_id], :order => "date asc" ).each do |r|
     r.diff_days = r.date - date_last

     # analog meters will reset to 0000 when after reaching 9999.  Difficult to recognise automatically, but will try
     if r.counter < counter_last and r.counter < 9000 and counter_last >9000:
       r.diff_counter = r.counter+ 10000- counter_last
     elsif
       r.diff_counter = r.counter - counter_last
     end

     if r.type == "ElectricityReading"
       co2_factor = 0.43
     elsif  r.type == "GasReading"
       co2_factor = 2.28
     elsif r.type == "WaterReading"
       co2_factor = 5
     else
       co2_factor = nil
     end

     r.diff_co2 = r.diff_counter.to_f * co2_factor
     #if this is the first record, the denominator will be zero, so we'll skip it
     unless r.diff_days == 0
       r.diff_counter_days = r.diff_counter.to_f / r.diff_days.to_f
       r.diff_co2_days = r.diff_counter_days * co2_factor
    end
    logger.debug r.type
     r.save
      counter_last = r.counter
      date_last =r.date

    end

  end


## Sub Class
class ElectricityReading < Reading


## Controller
def create_electricity_reading
     @reading = ElectricityReading.new(params[:reading])
    respond_to do |format|
     if @reading.save
        flash[:notice] = "created E-reading ok"
        format.html { redirect_to :action =>"index" }
      else
        format.html { render :action => "new" }
      end
    end
   ElectricityReading.all_diff(params[:reading][:unit_id])
  end