[Newbie] ActiveRecord: Calculate maximum of calculated column

rubyonrails-talk
1

Hi there,

I recently started learning Ruby on Rails. I decided to learn Rails by programming some kind of small statistic application where I can document the fuel consumption of my car - i.e. everytime I'm at the gas station I log price and amount of the gas refuled and the distance travelled since the last refuelling.

One of my models looks like this:

# start class Refuelling < ActiveRecord::Base   belongs_to :car

  def consumption     liter/kilometer * 100   end

  def liter_price     price/liter   end

  def type_of_tires     if tires       "winter tires"     else       "normal tires"     end   end end # end

In my view I use this like this (simplyfied):

# start <% for r in @refuelling %> <%=h r.created_at.to_date %> <%=h r.kilometer %> <%=h r.liter %> <%=h r.price %> <%=h r.consumption %> <%=h r.liter_price %> <%=h r.type_of_tires %> <% end %> # end

Now I want to calculate the max, min, average of this consumption method, but I do not know how. I tried creating a method like ...

# start def max   maximum(consumption) end # end

... in my model but that did not work.

Any hints on how to accomplish this? Thanks.

Max

2

Hi there, # start def max maximum(consumption) end # end

The activerecord maximum function maps straight to a sql MAX(), so can't work on a calculated column like this. In general, unless you are willing to load all the corresponding ruby objects into memory and iterate over them, pretty much the only thing you can do is write a query that will do the whole thing database side.

You can do something like Refuelling.maximum('liter / distance') but there's not a straightforward way of avoiding defining the calculation in 2 places.

Fred